This corrects an oversight by me in 20729324, which made
ProcArrayClearTransaction() increment xactCompletionCount. That requires an
exclusive lock, obviously.

There's other approaches that avoid the exclusive acquisition, but given that a
2PC commit is fairly heavyweight, it doesn't seem worth doing so. I've not been
able to measure a performance difference, unsurprisingly.  I did add a
comment documenting that we could do so, should it ever become a bottleneck.
Reported-By: Tom Lane <tgl@sss.pgh.pa.us>
Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/1355915.1597794204@sss.pgh.pa.us