Make sure libcrypto threadsafety callbacks are properly set up

Multithreaded programs using libcrypto (part of OpenSSL) need to set up
callbacks to ensure safe execution. Both Python and libpq set up those
callbacks, which might lead to a conflict.

To avoid leaving dangling function pointers when being unloaded, libpq sets up
and removes the callbacks every time a SSL connection it opened and closed. If
another Python thread is performing unrelated SSL operations (like connecting
to a HTTPS server), this might lead to deadlocks, as described in
http://www.postgresql.org/message-id/871tlzrlkq.fsf@wulczer.org

Even if the problem will be remediated in libpq, it's still useful to have it
fixed in psycopg2. The solution is to use Python's own libcrypto callbacks and
completely disable handling them in libpq.

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