Code review for log_lock_waits patch. Don't try to issue log messages from

Code review for log_lock_waits patch.  Don't try to issue log messages from
within a signal handler (this might be safe given the relatively narrow code
range in which the interrupt is enabled, but it seems awfully risky); do issue
more informative log messages that tell what is being waited for and the exact
length of the wait; minor other code cleanup.  Greg Stark and Tom Lane