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>

Cargo.toml

@@ -90,6 +90,7 @@ members = [
     "validator_client",
     "validator_client/slashing_protection",
 ]
+resolver = "2"
 
 [patch]
 [patch.crates-io]