Fix performance problems in TOAST compressor. The management of

search lists was broken in such a way that only the most recent
instance of a given hash code would ever be searched, thus possibly
missing longer matches further back.  Fixing this gave 5 to 10%
compression improvement on some text test cases.  Additional small
tweaks to improve speed of inner loops a little bit.  There is no
compatibility issue created by this change, since the compressed data
format and decompression algorithm don't change.