Add background file logging (#2762)

## Issue Addressed

Closes #1996 

## Proposed Changes

Run a second `Logger` via `sloggers` which logs to a file in the background with:
- separate `debug-level` for background and terminal logging
- the ability to limit log size
- rotation through a customizable number of log files
- an option to compress old log files (`.gz` format)

Add the following new CLI flags:
- `--logfile-debug-level`: The debug level of the log files
- `--logfile-max-size`: The maximum size of each log file
- `--logfile-max-number`: The number of old log files to store
- `--logfile-compress`: Whether to compress old log files

By default background logging uses the `debug` log level and saves logfiles to:
- Beacon Node:  `$HOME/.lighthouse/$network/beacon/logs/beacon.log`
- Validator Client:  `$HOME/.lighthouse/$network/validators/logs/validator.log`

Or, when using the `--datadir` flag:
`$datadir/beacon/logs/beacon.log` and `$datadir/validators/logs/validator.log`

Once rotated, old logs are stored like so: `beacon.log.1`, `beacon.log.2` etc. 
> Note: `beacon.log.1` is always newer than `beacon.log.2`.

## Additional Info

Currently the default value of `--logfile-max-size` is 200 (MB) and `--logfile-max-number` is 5.
This means that the maximum storage space that the logs will take up by default is 1.2GB. 
(200MB x 5 from old log files + <200MB the current logfile being written to)
Happy to adjust these default values to whatever people think is appropriate. 

It's also worth noting that when logging to a file, we lose our custom `slog` formatting. This means the logfile logs look like this:
```
Oct 27 16:02:50.305 INFO Lighthouse started, version: Lighthouse/v2.0.1-8edd9d4+, module: lighthouse:413
Oct 27 16:02:50.305 INFO Configured for network, name: prater, module: lighthouse:414
```

testing/simulator/src/eth1_sim.rs

@@ -6,8 +6,8 @@ use eth1::{DEFAULT_CHAIN_ID, DEFAULT_NETWORK_ID};
 use eth1_test_rig::GanacheEth1Instance;
 use futures::prelude::*;
 use node_test_rig::{
-    environment::EnvironmentBuilder, testing_client_config, testing_validator_config,
-    ClientGenesis, ValidatorFiles,
+    environment::{EnvironmentBuilder, LoggerConfig},
+    testing_client_config, testing_validator_config, ClientGenesis, ValidatorFiles,
 };
 use rayon::prelude::*;
 use sensitive_url::SensitiveUrl;
@@ -53,7 +53,15 @@ pub fn run_eth1_sim(matches: &ArgMatches) -> Result<(), String> {
     let log_format = None;
 
     let mut env = EnvironmentBuilder::minimal()
-        .async_logger(log_level, log_format)?
+        .initialize_logger(LoggerConfig {
+            path: None,
+            debug_level: log_level,
+            logfile_debug_level: "debug",
+            log_format,
+            max_log_size: 0,
+            max_log_number: 0,
+            compression: false,
+        })?
         .multi_threaded_tokio_runtime()?
         .build()?;
 

testing/simulator/src/no_eth1_sim.rs

@@ -2,8 +2,8 @@ use crate::{checks, LocalNetwork};
 use clap::ArgMatches;
 use futures::prelude::*;
 use node_test_rig::{
-    environment::EnvironmentBuilder, testing_client_config, testing_validator_config,
-    ClientGenesis, ValidatorFiles,
+    environment::{EnvironmentBuilder, LoggerConfig},
+    testing_client_config, testing_validator_config, ClientGenesis, ValidatorFiles,
 };
 use rayon::prelude::*;
 use std::cmp::max;
@@ -45,7 +45,15 @@ pub fn run_no_eth1_sim(matches: &ArgMatches) -> Result<(), String> {
     let log_format = None;
 
     let mut env = EnvironmentBuilder::mainnet()
-        .async_logger(log_level, log_format)?
+        .initialize_logger(LoggerConfig {
+            path: None,
+            debug_level: log_level,
+            logfile_debug_level: "debug",
+            log_format,
+            max_log_size: 0,
+            max_log_number: 0,
+            compression: false,
+        })?
         .multi_threaded_tokio_runtime()?
         .build()?;
 

testing/simulator/src/sync_sim.rs

@@ -3,7 +3,8 @@ use crate::local_network::LocalNetwork;
 use clap::ArgMatches;
 use futures::prelude::*;
 use node_test_rig::{
-    environment::EnvironmentBuilder, testing_client_config, ClientGenesis, ValidatorFiles,
+    environment::{EnvironmentBuilder, LoggerConfig},
+    testing_client_config, ClientGenesis, ValidatorFiles,
 };
 use node_test_rig::{testing_validator_config, ClientConfig};
 use std::cmp::max;
@@ -45,7 +46,15 @@ fn syncing_sim(
     log_format: Option<&str>,
 ) -> Result<(), String> {
     let mut env = EnvironmentBuilder::minimal()
-        .async_logger(log_level, log_format)?
+        .initialize_logger(LoggerConfig {
+            path: None,
+            debug_level: log_level,
+            logfile_debug_level: "debug",
+            log_format,
+            max_log_size: 0,
+            max_log_number: 0,
+            compression: false,
+        })?
         .multi_threaded_tokio_runtime()?
         .build()?;