quick-read: prune cache on write/[f]truncate

Cache needs to be pruned on write and [f]truncate. The lack of this
is causing Samba ping-pong test to return wierd 'data increment' values
during startup.

Change-Id: I9cd6a839bcd02de738d78638211b78f382f58e0a
BUG: 953694
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5033
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>