HBASE-12680 Move signal related code from ClusterManager to HBaseClusterManager and change ClusterManager to an interface

Summary: The reason of this change is to make us write implementation of ClusterManager not using ssh/unix signals.

Test Plan: The compilation is OK.

Reviewers: eclark, manukranthk

Differential Revision: https://reviews.facebook.net/D30201

Signed-off-by: Elliott Clark <eclark@apache.org>
HBASE-12692 NPE from SnapshotManager#stop (Ashish Singhi)
HBASE-12683 Compilation with hadoop-2.7.0-SNAPSHOT is broken
HBASE-12685 TestSplitLogManager#testLogFilesAreArchived sometimes times out due to race condition
HBASE-12666 TestAssignmentManager hanging; add timeouts
HBASE-12493 Make User and TokenUtil public

Conflicts:
	hbase-common/src/main/java/org/apache/hadoop/hbase/security/User.java
	hbase-server/src/main/java/org/apache/hadoop/hbase/mapred/TableMapReduceUtil.java
	hbase-server/src/main/java/org/apache/hadoop/hbase/mapreduce/TableMapReduceUtil.java
	hbase-server/src/main/java/org/apache/hadoop/hbase/security/token/TokenUtil.java
	hbase-server/src/test/java/org/apache/hadoop/hbase/security/token/TestTokenAuthentication.java
HBASE-12348 preModifyColumn and preDeleteColumn in AC denies user to perform its operation though it has required rights

Signed-off-by: Andrew Purtell <apurtell@apache.org>
HBASE-12686 Failures in split before PONR not clearing the daughter regions from regions in transition during rollback (Vandana Ayyalasomayajula)
HBASE-12696 Possible NPE in HRegionFileSystem#splitStoreFile when skipStoreFileRangeCheck in splitPolicy return true(Rajeshbabu)
HBASE-12493 User class should provide a way to re-use existing token - addendum for hadoop-1
Update CHANGES.txt for 0.98.9RC1