Remove QueueLock

[nicktindall]

The QueueLock was used to prevent writes to the queue while a back-fill was in progress. A better approach was implemented via the replication heartbeat, this indicates to potential writers that the replicator has completed all back-fills and it is safe to write. Writers can choose whether to only write when safe, or write at other times when they know it to be safe.

[hft-team-city]

Released in Chronicle-Queue-5.25ea0, BOM-2.25ea0