feat: add async cache for segment indexes

[jeqo]

Indexes are currently cached on broker-side but with a synchronous cache that is not able to source value when interrupted.
This new cache will fetch index asynchronously, and retries would have chances to find the value sourced already.

[ivanyu]

Do these two need to be higher-order functions?

Suggested change

	public RemovalListener<SegmentIndexKey, byte[]> removalListener() {
	return (key, content, cause) -> log.debug("Deleted cached value for key {} from cache."
	+ " The reason of the deletion is {}", key, cause);
	}
	public Weigher<SegmentIndexKey, byte[]> weigher() {
	return (key, value) -> value.length;
	}
	private static void removalListener(final SegmentIndexKey key, final byte[] value, RemovalCause cause) {
	log.debug("Deleted cached value for key {} from cache. The reason of the deletion is {}", key, cause);
	}
	private static int weigher(final SegmentIndexKey key, byte[] value) {
	return value.length;
	}

[ivanyu]

It's worth adding a comment that this sleep is needed to spread away in time the creation (and eviction) of the cache entries, or something like this

[jeqo]

Good point, let's do the latter.

[jeqo]

@AnatolyPopov, falling back to using thread sleep (same as in ChunkCacheTest) as adding a stronger condition (e.g. waiting for cache to be empty) takes unknown amount of time github actions--unless there's a better approach here.

[ivanyu]

LGTM