glusterfsd: Remove 'top {read,write}-perf' thread

'volume top {read,write}-perf' were performed in a different thread on the brick
process, to prevent them from blocking the main polling thread. This was
essential earlier when the rpc actor functions were run in the epoll thread. Now
rpc actors can be launched on their own threads with the introduction of
synctask support for rpcsvc. The brick-op actors are now launched using this
feature, which makes the need for '{read,write}-perf' to be performed in a new
thread.

Change-Id: Idad7ab623f0bfb0c3a5cfae796c06e0fa3cff1b9
BUG: 764890
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/4510
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Reviewed-by: Anand Avati <avati@redhat.com>