CDRIVER-294 make mongoc-ssl thread safe

Use
* CRYPTO_set_locking_callback
* CRYPTO_set_id_callback

to make mongoc-ssl thread safe (openssl needs locking and thread id
primitives appropriate to your platform, we provide pthreads and win32).

hooks startup into mongoc_init and cleanup into mongoc_cleanup
Merge pull request #16 from hanumantmk/CDRIVER-294

CDRIVER-294 make mongoc-ssl thread safe