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>

common/malloc_utils/Cargo.toml

@@ -4,13 +4,21 @@ version = "0.1.0"
 authors = ["Paul Hauner <paul@paulhauner.com>"]
 edition = "2021"
 
-# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
-
 [dependencies]
 lighthouse_metrics = { path = "../lighthouse_metrics" }
 lazy_static = "1.4.0"
 libc = "0.2.79"
 parking_lot = "0.12.0"
+jemalloc-ctl = { version = "0.5.0", optional = true }
+
+# Jemalloc's background_threads feature requires Linux (pthreads).
+[target.'cfg(target_os = "linux")'.dependencies]
+jemallocator = { version = "0.5.0", optional = true, features = ["stats", "background_threads"] }
+
+[target.'cfg(not(target_os = "linux"))'.dependencies]
+jemallocator = { version = "0.5.0", optional = true, features = ["stats"] }
 
 [features]
 mallinfo2 = []
+jemalloc = ["jemallocator", "jemalloc-ctl"]
+jemalloc-profiling = ["jemallocator/profiling"]