add support for empty/blank tile detection in sqlite caches

also:
* refactor tile merging code to avoid some unnecessary image
  decodings/encodings when some of the merged tiles are blank
* don't create sqlite db for read-only connections (on tile_get or
  has_tile), return error if tileset is read-only
add a reslist to share sqlite connection to sqlite time db (#62)
add support for extent in timedimension lookups
update mapcache version number to -dev