fixup! Refactor flushAll by making use of vbucket_version
fixup! Refactor flushAll by making use of vbucket_version

There is a small window between hashtable clear and vbucket version increment.
Crash can happen if mutations are enqueued to flusher queue between this interval.
This commit makes increment of vbucket version and clearing of hashtable atomic by
holding a hashtable lock.

Signed-off-by: Sarath Lakshman <88fea7318180dc614b9a8286d9cfacb1e36761af@zynga.com>

zbase/ep-engine,zbase/ep-engine,zbase/ep-engine,zbase/ep-engine,zbase/ep-engine