HBASE-12736 Let MetaScanner recycle a given Connection (Jurriaan Mous)

Signed-off-by: stack <stack@apache.org>

Conflicts:
	hbase-server/src/test/java/org/apache/hadoop/hbase/master/TestRestartCluster.java
	hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestEndToEndSplitTransaction.java
	hbase-server/src/test/java/org/apache/hadoop/hbase/util/TestHBaseFsck.java
HBASE-12739 Avoid too large identifier of ZooKeeperWatcher (Liu Shaohui)