Adjust "pgstat wait timeout" message to be a translatable LOG message.

 Adjust "pgstat wait timeout" message to be a translatable LOG message.
 
 Per discussion, change the log level of this message to be LOG not WARNING.
 The main point of this change is to avoid causing buildfarm run failures
 when the stats collector is exceptionally slow to respond, which it not
 infrequently is on some of the smaller/slower buildfarm members.
 
 This change does lose notice to an interactive user when his stats query
 is looking at out-of-date stats, but the majority opinion (not necessarily
 that of yours truly) is that WARNING messages would probably not get
 noticed anyway on heavily loaded production systems.  A LOG message at
 least ensures that the problem is recorded somewhere where bulk auditing
 for the issue is possible.
 
 Also, instead of an untranslated "pgstat wait timeout" message, provide
 a translatable and hopefully more understandable message "using stale
 statistics instead of current ones because stats collector is not
 responding".  The original text was written hastily under the assumption
 that it would never really happen in practice, which we now know to be
 unduly optimistic.
 
 Back-patch to all active branches, since we've seen the buildfarm issue
 in all branches.