Retryable Batches

[gdbelvin]

The TrillianMap API needs to be retried in the event of any failure. TODO: add Trillian issue. This will greatly improve the reliability of the system and support recovery in the event of a failure.

There will be a series of small changes to support retryable batches:

• Switching to a ReadQueue API
• Read from Queue via min and max Timestamps
• Deprecating the push based Receiver API for mutations
• QueueMutation will write directly to a sharded, time ordered table.
• CommitBatch will store high and low watermarks for a batch
• ApplyBatch will read the mutation table at committed watermarks and apply the mutation function.
• PublishBatch will write the SignedMapHead to the Log of MapHeads