Pull the incremental_vacuum bug fix ([255f1eefa373153942c67b18b]) and the
R-tree segfault bug fix ([7f2f71cc9e3c39093f09231f44]) into the 3.6.23
branch.  Increase the version number to 3.6.23.3.
Backport the RTree implementation from the trunk into the 3.6.23 branch.
The code for the application-defined query boxes is still present but
is disabled.  The reason for this backport is to take advantage of recent
enhancements to robustness to database corruption.
Fix the amalgamation builder so that it works with the rtree updates of
the prior check-in.