Further improvements to pyopenssl sendall()

This doesn't currently do anything useful, because PyOpenSSL needs to
convert back to a string to pass to ffi. It doesn't hurt, though.

Fix performance of sendall() on large files.

OpenSSL will only write out 16K at once (max length of a single chunk of
plaintext data) before returning from SSL_write, so there's no point in trying
to pass more data than that to it at a time.  The subsequent copy of 'data'
ended up being quite expensive, though.

refactor assert_fingerprint further for efficiency and style