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>

lighthouse/src/main.rs

@@ -31,6 +31,14 @@ fn bls_library_name() -> &'static str {
     }
 }
 
+fn allocator_name() -> &'static str {
+    if cfg!(feature = "jemalloc") {
+        "jemalloc"
+    } else {
+        "system"
+    }
+}
+
 fn main() {
     // Enable backtraces unless a RUST_BACKTRACE value has already been explicitly provided.
     if std::env::var("RUST_BACKTRACE").is_err() {
@@ -51,10 +59,12 @@ fn main() {
                 "{}\n\
                  BLS library: {}\n\
                  SHA256 hardware acceleration: {}\n\
+                 Allocator: {}\n\
                  Specs: mainnet (true), minimal ({}), gnosis ({})",
                  VERSION.replace("Lighthouse/", ""),
                  bls_library_name(),
                  have_sha_extensions(),
+                 allocator_name(),
                  cfg!(feature = "spec-minimal"),
                  cfg!(feature = "gnosis"),
             ).as_str()