Introduce jemalloc-allocator feature flag (#6675)

* Introduce jemalloc-stats feature flag

* remove unneeded space

* Update node/overseer/src/lib.rs

Co-authored-by: Marcin S. <[email protected]>

* Update Cargo.toml

Co-authored-by: Marcin S. <[email protected]>

* revert making tikv-jemallocator depend on jemalloc-stats

* conditionally import memory_stats instead of using dead_code

* fix test via expllicit import

* Add jemalloc-stats feature to crates, propagate it from root

* Apply `jemalloc-stats` feature to prepare mem stats; small refactor

* effect changes recommended on PR

* Update node/overseer/src/metrics.rs

Co-authored-by: Marcin S. <[email protected]>

* fix compile error on in pipeline for linux. missing import

* Update node/overseer/src/lib.rs

Co-authored-by: Bastian Köcher <[email protected]>

* revert to defining collect_memory_stats inline

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Co-authored-by: Marcin S. <[email protected]>
Co-authored-by: Marcin S <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>