from the discussion in HBASE-12439 the wal was resulting slow.
- there is an error around the awake of the slotCond.await(), causing more wait then necessary
- ArrayBlockingQueue is dog slow, replace it with ConcurrentLinkedQueue
- roll the wal only if reaches a threshold (conf ops) to amortize the cost
- hsync() is used by default, when the normal wal is using just hflush() make it tunable via conf