HBASE-12535 NPE in WALFactory under contention for getInstance()

Signed-off-by: stack <stack@apache.org>
HBASE-12539 HFileLinkCleaner logs are uselessly noisy
HBASE-11939 Document Compressed Blockcache
HBASE-8572 Enhance delete_snapshot.rb to call snapshot deletion API with regex (Ashish Singhi)
HBASE-12464 meta table region assignment stuck in the FAILED_OPEN state due to region server not fully ready to serve (Stephen Jiang)
HBASE-11099 Two situations where we could open a region with smaller sequence number (Stephen Jiang)
HBASE-12516 Clean up QA Bot warnings

Signed-off-by: stack <stack@apache.org>
HBASE-12549 Fix TestAssignmentManagerOnCluster#testAssignRacingWithSSH() flaky test