Display the time when the process started waiting for the lock, in pg_locks, take 2

 Display the time when the process started waiting for the lock, in pg_locks, take 2
 
 This commit adds new column "waitstart" into pg_locks view. This column
 reports the time when the server process started waiting for the lock
 if the lock is not held. This information is useful, for example, when
 examining the amount of time to wait on a lock by subtracting
 "waitstart" in pg_locks from the current time, and identify the lock
 that the processes are waiting for very long.
 
 This feature uses the current time obtained for the deadlock timeout
 timer as "waitstart" (i.e., the time when this process started waiting
 for the lock). Since getting the current time newly can cause overhead,
 we reuse the already-obtained time to avoid that overhead.
 
 Note that "waitstart" is updated without holding the lock table's
 partition lock, to avoid the overhead by additional lock acquisition.
 This can cause "waitstart" in pg_locks to become NULL for a very short
 period of time after the wait started even though "granted" is false.
 This is OK in practice because we can assume that users are likely to
 look at "waitstart" when waiting for the lock for a long time.
 
 The first attempt of this patch (commit 3b733fcd04) caused the buildfarm
 member "rorqual" (built with --disable-atomics --disable-spinlocks) to report
 the failure of the regression test. It was reverted by commit 890d2182a2.
 The cause of this failure was that the atomic variable for "waitstart"
 in the dummy process entry created at the end of prepare transaction was
 not initialized. This second attempt fixes that issue.
 
 Bump catalog version.
 
 Author: Atsushi Torikoshi
 Reviewed-by: Ian Lawrence Barwick, Robert Haas, Justin Pryzby, Fujii Masao
 Discussion: https://postgr.es/m/a96013dc51cdc56b2a2b84fa8a16a993@oss.nttdata.com