synctask: introduce synclocks for co-operative locking

This patch introduces a synclocks - co-operative locks for synctasks.
Synctasks yield themselves when a lock cannot be acquired at the time
of the lock call, and the unlocker will wake the yielded locker at
the time of unlock.

The implementation is safe in a multi-threaded syncenv framework.

It is also safe for sharing the lock between non-synctasks. i.e, the
same lock can be used for synchronization between a synctask and
a regular thread. In such a situation, waiting synctasks will yield
themselves while non-synctasks will sleep on a cond variable. The
unlocker (which could be either a synctask or a regular thread) will
wake up any type of lock waiter (synctask or regular).

Usage:

    Declaration and Initialization
    ------------------------------

    synclock_t lock;

    ret = synclock_init (&lock);
    if (ret) {
        /* lock could not be allocated */
    }

   Locking and non-blocking lock attempt
   -------------------------------------

   ret = synclock_trylock (&lock);
   if (ret && (errno == EBUSY)) {
      /* lock is held by someone else */
      return;
   }

   synclock_lock (&lock);
   {
      /* critical section */
   }
   synclock_unlock (&lock);

BUG: 763820
Change-Id: I23066f7b66b41d3d9fb2311fdaca333e98dd7442
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Original-author: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/4830
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>