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/src/config.rs

@@ -1,5 +1,5 @@
 use clap::ArgMatches;
-use clap_utils::BAD_TESTNET_DIR_MESSAGE;
+use clap_utils::{flags::DISABLE_MALLOC_TUNING_FLAG, BAD_TESTNET_DIR_MESSAGE};
 use client::{ClientConfig, ClientGenesis};
 use directory::{DEFAULT_BEACON_NODE_DIR, DEFAULT_NETWORK_DIR, DEFAULT_ROOT_DIR};
 use eth2_libp2p::{multiaddr::Protocol, Enr, Multiaddr, NetworkConfig, PeerIdSerialized};
@@ -156,6 +156,11 @@ pub fn get_config<E: EthSpec>(
         );
     }
 
+    // Do not scrape for malloc metrics if we've disabled tuning malloc as it may cause panics.
+    if cli_args.is_present(DISABLE_MALLOC_TUNING_FLAG) {
+        client_config.http_metrics.allocator_metrics_enabled = false;
+    }
+
     /*
      * Eth1
      */