Tune GNU malloc (#2299)

## Issue Addressed

NA

## Proposed Changes

Modify the configuration of [GNU malloc](https://www.gnu.org/software/libc/manual/html_node/The-GNU-Allocator.html) to reduce memory footprint.

- Set `M_ARENA_MAX` to 4.
    - This reduces memory fragmentation at the cost of contention between threads.
- Set `M_MMAP_THRESHOLD` to 2mb
    - This means that any allocation >= 2mb is allocated via an anonymous mmap, instead of on the heap/arena. This reduces memory fragmentation since we don't need to keep growing the heap to find big contiguous slabs of free memory.
- ~~Run `malloc_trim` every 60 seconds.~~
    - ~~This shaves unused memory from the top of the heap, preventing the heap from constantly growing.~~
    - Removed, see: https://github.com/sigp/lighthouse/pull/2299#issuecomment-825322646

*Note: this only provides memory savings on the Linux (glibc) platform.*
    
## Additional Info

I'm going to close #2288 in favor of this for the following reasons:

- I've managed to get the memory footprint *smaller* here than with jemalloc.
- This PR seems to be less of a dramatic change than bringing in the jemalloc dep.
- The changes in this PR are strictly runtime changes, so we can create CLI flags which disable them completely. Since this change is wide-reaching and complex, it's nice to have an easy "escape hatch" if there are undesired consequences.

## TODO

- [x] Allow configuration via CLI flags
- [x] Test on Mac
- [x] Test on RasPi.
- [x] Determine if GNU malloc is present?
    - I'm not quite sure how to detect for glibc.. This issue suggests we can't really: https://github.com/rust-lang/rust/issues/33244
- [x] Make a clear argument regarding the affect of this on CPU utilization.
- [x] Test with higher `M_ARENA_MAX` values.
- [x] Test with longer trim intervals
- [x] Add some stats about memory savings
- [x] Remove `malloc_trim` calls & code

beacon_node/http_metrics/src/lib.rs

@@ -49,6 +49,7 @@ pub struct Config {
     pub listen_addr: Ipv4Addr,
     pub listen_port: u16,
     pub allow_origin: Option<String>,
+    pub allocator_metrics_enabled: bool,
 }
 
 impl Default for Config {
@@ -58,6 +59,7 @@ impl Default for Config {
             listen_addr: Ipv4Addr::new(127, 0, 0, 1),
             listen_port: 5054,
             allow_origin: None,
+            allocator_metrics_enabled: true,
         }
     }
 }

beacon_node/http_metrics/src/metrics.rs

@@ -1,6 +1,7 @@
 use crate::Context;
 use beacon_chain::BeaconChainTypes;
 use lighthouse_metrics::{Encoder, TextEncoder};
+use malloc_utils::scrape_allocator_metrics;
 
 pub use lighthouse_metrics::*;
 
@@ -41,6 +42,12 @@ pub fn gather_prometheus_metrics<T: BeaconChainTypes>(
 
     warp_utils::metrics::scrape_health_metrics();
 
+    // It's important to ensure these metrics are explicitly enabled in the case that users aren't
+    // using glibc and this function causes panics.
+    if ctx.config.allocator_metrics_enabled {
+        scrape_allocator_metrics();
+    }
+
     encoder
         .encode(&lighthouse_metrics::gather(), &mut buffer)
         .unwrap();