Refactor shard recovery from anonymous class to ShardRecoveryHandler

Previously the bulk of our shard recovery code was in a 300-line
anonymous class in `RecoverySource`. This made it difficult to find and
more difficult to read.

This factors out that code into a `ShardRecoveryHandler` class, adding
javadocs for each function and phase of the recovery, as well as
comments explaining some of the more esoteric functions performed during
recovery.

It's hoped that this will help more people understand Elasticsearch's
recovery procedure.

No *major* functionality has changed, only typo corrections, some minor
allocation improvements and logging clarification changes.
I think this should be final