Fix requestid_counter reset to fit int32

This pull request fixes #24 (Can't connect to DB after integer exceeds 32 bits).

Root cause is that requestid_counter (in mongodb_app.erl) should fit int32 (signed!), otherwise after a reasonable period of time a high load application crashes reaching the limit. Happened twice at my deployment. And happened to #24 reporter few times.

As a bonus I updated mongo.erl to be able to compile it on Erlang R16.