[branch-1] Backport improvements to FSHLog from branch-2

From c96b642f15d (HBASE-15265 Implement an asynchronous FSHLog)

• Better formatting; use { } instead of dangling ifs, elses, etc. in RingBufferEventHandler.

From b1269ec57ff (HBASE-19811 Fix findbugs and error-prone warnings in hbase-server (branch-2))

• Change syncFuturesCount in RingBufferEventHandler from 'volatile int' to AtomicInteger.
  'volatile' is insufficient for multithreaded access.

From afc1746757f (HBASE-24034 [Flakey Tests] A couple of fixes and cleanups)

• Make a local copy of takeSyncFuture after we get it in SyncRunner#run.
  This looks like a workaround for a JIT/compiler bug in some Java versions. (SCARY)