Enable jemalloc for benchmarks and tests

[erikgrinaker]

Currently, Jemalloc is enabled (for Pageserver, Proxy, and Safekeeper) in the binary (e.g. bin/pageserver.rs), not in the crate itself (i.e. lib.rs). This means that benchmarks and tests use the standard allocator, which in particular may affect benchmark results.

We should enable Jemalloc crate-wide in lib.rs itself, but only via a feature or cfg attribute that's enabled by the binaries and tests/benchmarks. Otherwise, loading the crate will implicitly enable Jemalloc for crate consumers, and conflict with other crates that also change the allocator.