Parser throws NullPointerException when Filter aggregation clause is empty.
Added Junit test that recreates the error and fixed FilterParser to default to using a MatchAllDocsFilter if the requested filter clause is left empty.
Also added fix and test for the Filters (with an "s") aggregation.

Closes #8438
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.

Conflicts:
	src/main/java/org/elasticsearch/indices/recovery/RecoverySource.java
Mark ShardRecoveryShandler class as final
Test: CorruptedFileTest.testCorruptionOnNetworkLayer used node settings when creating an index

Test used `indices.recovery.concurrent_streams` when creating an index but this is a node setting. Moved it to the node settings and added similar settings to speed up concurrent recoveries.

Also fixed a misleading log message in ShardRecoveryHandler when logging a remove corruption
[RECOVERY] Allow to cancle recovery sources when shards are closed

Today recovery sources are not cancled if a shard is closed. The recovery target
is already cancled when shards are closed but we should also cleanup and cancel
the sources side since it holds on to shard locks / references until it's closed.