Use annotations to reduce instability of isolation-test results.

 Use annotations to reduce instability of isolation-test results.
 
 We've long contended with isolation test results that aren't entirely
 stable.  Some test scripts insert long delays to try to force stable
 results, which is not terribly desirable; but other erratic failure
 modes remain, causing unrepeatable buildfarm failures.  I've spent a
 fair amount of time trying to solve this by improving the server-side
 support code, without much success: that way is fundamentally unable
 to cope with diffs that stem from chance ordering of arrival of
 messages from different server processes.
 
 We can improve matters on the client side, however, by annotating
 the test scripts themselves to show the desired reporting order
 of events that might occur in different orders.  This patch adds
 three types of annotations to deal with (a) test steps that might or
 might not complete their waits before the isolationtester can see them
 waiting; (b) test steps in different sessions that can legitimately
 complete in either order; and (c) NOTIFY messages that might arrive
 before or after the completion of a step in another session.  We might
 need more annotation types later, but this seems to be enough to deal
 with the instabilities we've seen in the buildfarm.  It also lets us
 get rid of all the long delays that were previously used, cutting more
 than a minute off the runtime of the isolation tests.
 
 Back-patch to all supported branches, because the buildfarm
 instabilities affect all the branches, and because it seems desirable
 to keep isolationtester's capabilities the same across all branches
 to simplify possible future back-patching of tests.
 
 Discussion: https://postgr.es/m/327948.1623725828@sss.pgh.pa.us