Speedups

These commits implement the suggestions in https://jira.mongodb.org/browse/PERL-129, for a 65% speedup for inserts and 14% speedup for fetching (using the insert.pl script in the ticket).
This kinda smells to me. Usually any sign of `<=` or `len + 1` when dealing with buffer management means something potentially bad could be lurking. For example, if `perl_mongo_resize_buf` simply resizes `buf` to `len+1` then this will always be true - even right after a call to `perl_mongo_resize_buf`.