Release notes for 10.1, 9.6.6, 9.5.10, 9.4.15, 9.3.20, 9.2.24.

Release notes for 10.1, 9.6.6, 9.5.10, 9.4.15, 9.3.20, 9.2.24.

In the v10 branch, also back-patch the effects of 1ff01b390 and c29c57890
on these files, to reduce future maintenance issues.  (I'd do it further
back, except that the 9.X branches differ anyway due to xlog-to-wal
link tag renaming.)