LU-1669 xxx: Add waitq to range_lock structure

Added a waitq to the range_lock structure. This allows threads to sleep
on the waitq when a conflicting lock is found, and these threads can
then be woken up when the offending lock is removed.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Change-Id: I5858c5018d8087e6e9cc27d38ec4ec5562f1cec0