better cache sizing in unbuffered sequential mode

Previously we only ever had a single tile of cache in unbuffered mode.
This could break with many threads, if thread requests crossed tile
boundaries.

Now have one tile per thread, plus a bit for slop.
Merge remote-tracking branch 'origin/7.36'

Conflicts:
	ChangeLog
	configure.ac
allow larger tile_size in dzsave

bump the tile_size limit up, thanks Petter Ranefall
Merge branch '7.36'