Remove the stats block in SSL_CTX.

Within the library, only ssl_update_cache read them, so add a dedicated field
to replace that use.

The APIs have a handful of uninteresting callers so I've left them in for now,
but they now always return zero.

Change-Id: Ie4e36fd4ab18f9bff544541d042bf3c098a46933
Reviewed-on: https://boringssl-review.googlesource.com/4101
Reviewed-by: Adam Langley <agl@google.com>