ZkLedgerUnderreplicationManager: cache responses / reduce number of zk calls by isLedgerReplicationEnabled()

[dlg99]

It looks like ZkLedgerUnderreplicationManager.isLedgerReplicationEnabled() can cache the state and either use watcher (see notifyLedgerReplicationEnabled) + enableLedgerReplication/disableLedgerReplication or just some time interval (10sec or so) to cache the value for.

AFAICT Auditor.checkAllLedgers() calls it rather frequently

[horizonzy]

good idea.