Rewrite gather-write patch into something less obviously bolted on

after the fact.  Fix bug with incorrect test for whether we are at end
of logfile segment.  Arrange for writes triggered by XLogInsert's
is-cache-more-than-half-full test to synchronize with the cache boundaries,
so that in long transactions we tend to write alternating halves of the
cache rather than randomly chosen portions of it; this saves one more
write syscall per cache load.