improve consensus download decompress performance
Reported by starlight.2015q3@binnacle.cx to tor-dev@
"Currently tor_gzip_uncompress() starts with an assumed 50% compression guess. Typical consensus document compression is slightly better than 65% and this assumption results in at least one realloc() and sometimes two realloc() calls during decompression.
Adjust the starting assumption to 75% compression to eliminate the cost of the realloc()."