Use per-table merge lock in Manager instead of single lock

[dlmarion]

This change introduces a per-table merge lock instead of a single lock to protect reading/writing to ZooKeeper.

Closes #6374

[ctubbsii]

Could use a Caffeine cache to handle this. The key could also be a hash of the TableId, if you wanted to stripe this, but less aggressively than one per table.

[ctubbsii]

Suggested change

	private final MergeLocks mergeLocks = new MergeLocks();
	private final LoadingCache<TableId,ReentrantLock> mergeLocks = Caffeine.newBuilder().weakValues()
	.scheduler(Scheduler.systemScheduler()).build(k -> new ReentrantLock());

My main concern with this is that the TableId in the keys will prevent cleanup of the TableId.cache weak values. The scheduler is supposed to help with cleanup of the keys whose values have been garbage collected, but it may also be a good idea to use String (tableId.canonical()) for the key.

[ctubbsii]

Suggested change

	private final class MergeLocks {
	private final Object lock = new Object();
	private Map<TableId,ReentrantLock> lockStorage = new HashMap<TableId,ReentrantLock>();
	private ReentrantLock getLock(TableId tid) {
	synchronized (lock) {
	return lockStorage.computeIfAbsent(tid, k -> new ReentrantLock(true));
	}
	}
	private void cleanup() {
	synchronized (lock) {
	Set<TableId> removals = new HashSet<>();
	for (Entry<TableId,ReentrantLock> e : lockStorage.entrySet()) {
	if (!getContext().tableNodeExists(e.getKey()) && !e.getValue().isLocked()) {
	removals.add(e.getKey());
	}
	}
	removals.forEach(lockStorage::remove);
	}
	}
	}

[ctubbsii]

Suggested change

	final ReentrantLock l = mergeLocks.getLock(tableId);
	final ReentrantLock l = mergeLocks.get(tableId);

[ctubbsii]

No need for this cleanup code with the weakValues cache's scheduler.