|SSL_CTX| and |X509_STORE| have grown their own locks. Several static
locks have been added to hack around not being able to use a
|CRYPTO_once_t| in public headers. Lastly, support for calling
|SSL_CTX_set_generate_session_id| concurrently with active connections
has been removed. No other property of an |SSL_CTX| works like that.

Change-Id: Iff5fe3ee3fdd6ea9c9daee96f850b107ad8a6bca
Reviewed-on: https://boringssl-review.googlesource.com/4775
Reviewed-by: Adam Langley <agl@google.com>