HBASE-7929 Reapply hbase-7507 'Make memstore flush be able to retry after exception' to 0.94 branch. (Original patch by chunhui shen)

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.94@1467121 13f79535-47bb-0310-9956-ffa450edef68
HBASE-8303. Increase the test timeout to 60s when they are less than 20s

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.94@1467157 13f79535-47bb-0310-9956-ffa450edef68