Allow ConditionVariable[PrepareTo]Sleep to auto-switch between CVs.

The original coding here insisted that callers manually cancel any prepared
sleep for one condition variable before starting a sleep on another one.
While that's not a huge burden today, it seems like a gotcha that will bite
us in future if the use of condition variables increases; anything we can
do to make the use of this API simpler and more robust is attractive.
Hence, allow these functions to automatically switch their attention to
a different CV when required.  This is safe for the same reason it was OK
for commit aced5a92b to let a broadcast operation cancel any prepared CV
sleep: whenever we return to the other test-and-sleep loop, we will
automatically re-prepare that CV, paying at most an extra test of that
loop's exit condition.

Back-patch to v10 where condition variables were introduced.  Ordinarily
we would probably not back-patch a change like this, but since it does not
invalidate any coding pattern that was legal before, it seems safe enough.
Furthermore, there's an open bug in replorigin_drop() for which the
simplest fix requires this.  Even if we chose to fix that in some more
complicated way, the hazard would remain that we might back-patch some
other bug fix that requires this behavior.

Patch by me, reviewed by Thomas Munro.

Discussion: https://postgr.es/m/2437.1515368316@sss.pgh.pa.us

src/include/storage/condition_variable.h

@@ -40,9 +40,7 @@ extern void ConditionVariableInit(ConditionVariable *cv);
  * ConditionVariableSleep.  Spurious wakeups are possible, but should be
  * infrequent.  After exiting the loop, ConditionVariableCancelSleep must
  * be called to ensure that the process is no longer in the wait list for
- * the condition variable.  Only one condition variable can be used at a
- * time, ie, ConditionVariableCancelSleep must be called before any attempt
- * is made to sleep on a different condition variable.
+ * the condition variable.
  */
 extern void ConditionVariableSleep(ConditionVariable *cv, uint32 wait_event_info);
 extern void ConditionVariableCancelSleep(void);