Switch allocator to jemalloc (#3697)

## Proposed Changes

Another `tree-states` motivated PR, this adds `jemalloc` as the default allocator, with an option to use the system allocator by compiling with `FEATURES="" make`.

- [x] Metrics
- [x] Test on Windows
- [x] Test on macOS
- [x] Test with `musl`
- [x] Metrics dashboard on `lighthouse-metrics` (https://github.com/sigp/lighthouse-metrics/pull/37)


Co-authored-by: Michael Sproul <micsproul@gmail.com>

book/src/installation-source.md

@@ -64,6 +64,7 @@ choco install protoc
 These dependencies are for compiling Lighthouse natively on Windows. Lighthouse can also run
 successfully under the [Windows Subsystem for Linux (WSL)][WSL]. If using Ubuntu under WSL, you
 should follow the instructions for Ubuntu listed in the [Dependencies (Ubuntu)](#ubuntu) section.
+
 [WSL]: https://docs.microsoft.com/en-us/windows/wsl/about
 
 ## Build Lighthouse
@@ -128,8 +129,12 @@ Commonly used features include:
 * `gnosis`: support for the Gnosis Beacon Chain.
 * `portable`: support for legacy hardware.
 * `modern`: support for exclusively modern hardware.
-* `slasher-mdbx`: support for the MDBX slasher backend (enabled by default).
+* `slasher-mdbx`: support for the MDBX slasher backend. Enabled by default.
 * `slasher-lmdb`: support for the LMDB slasher backend.
+* `jemalloc`: use [`jemalloc`][jemalloc] to allocate memory. Enabled by default on Linux and macOS.
+  Not supported on Windows.
+
+[jemalloc]: https://jemalloc.net/
 
 ## Compilation Profiles