Improve scalability of WAL insertions.

This patch replaces WALInsertLock with a number of WAL insertion slots,
allowing multiple backends to insert WAL records to the WAL buffers
concurrently. This is particularly useful for parallel loading large amounts
of data on a system with many CPUs.

This has one user-visible change: switching to a new WAL segment with
pg_switch_xlog() now fills the remaining unused portion of the segment with
zeros. This potentially adds some overhead, but it has been a very common
practice by DBA's to clear the "tail" of the segment with an external
pg_clearxlogtail utility anyway, to make the WAL files compress better.
With this patch, it's no longer necessary to do that.

This patch adds a new GUC, xloginsert_slots, to tune the number of WAL
insertion slots. Performance testing suggests that the default, 8, works
pretty well for all kinds of worklods, but I left the GUC in place to allow
others with different hardware to test that easily. We might want to remove
that before release.

Reviewed by Andres Freund.
Fix pg_test_fsync, broken by xloginsert scaling patch.

I didn't realize that ALIGNOF_XLOG_BUFFER was used in pg_test_fsync.
Fix Windows build.

Was broken by my xloginsert scaling patch. XLogCtl global variable needs
to be initialized in each process, as it's not inherited by fork() on
Windows.
Fix mention of htup.h in pageinspect docs

It's htup_details.h now.

Jeff Janes
Fix bool abuse

path_encode's "closed" argument used to take three values: TRUE, FALSE,
or -1, while being of type bool.  Replace that with a three-valued enum
for more clarity.