Fix indefinite hang in WAL.writeToLog
Fix indefinite hang in WAL.writeToLog

There was a race in the WAL writeToLog and scheduleSync which could
lead to a writing goroutine blocking indefinitely on its syncErr channel.

The issue was that the clearing of the syncCount happenend after the
wal was unlock.  If a goroutine was able to lock, write and call scheduleSync
before the existing scheduleSync goroutine returns and ran the defer to
clear the syncCount, then a new scheduleSync goroutine would not get started.
This left the writing goroutine block with nothing to signal it.

While in this state, a RLock on the engine was held.  If a Lock was requested
on the engine during this time, all future writes and queries would block waiting
on the blocked wal writer.

The fix is to move the atomic clearing of syncCount before the Lock is released.

runner-mei/influxdb,runner-mei/influxdb,runner-mei/influxdb,runner-mei/influxdb