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
```

lcli/src/main.rs

@@ -16,7 +16,7 @@ mod transition_blocks;
 
 use clap::{App, Arg, ArgMatches, SubCommand};
 use clap_utils::parse_path_with_default_in_home_dir;
-use environment::EnvironmentBuilder;
+use environment::{EnvironmentBuilder, LoggerConfig};
 use parse_ssz::run_parse_ssz;
 use std::path::PathBuf;
 use std::process;
@@ -584,8 +584,16 @@ fn run<T: EthSpec>(
     let env = env_builder
         .multi_threaded_tokio_runtime()
         .map_err(|e| format!("should start tokio runtime: {:?}", e))?
-        .async_logger("trace", None)
-        .map_err(|e| format!("should start null logger: {:?}", e))?
+        .initialize_logger(LoggerConfig {
+            path: None,
+            debug_level: "trace",
+            logfile_debug_level: "trace",
+            log_format: None,
+            max_log_size: 0,
+            max_log_number: 0,
+            compression: false,
+        })
+        .map_err(|e| format!("should start logger: {:?}", e))?
         .build()
         .map_err(|e| format!("should build env: {:?}", e))?;