The internal session cache is keyed on session ID, so this is completely
useless for clients (indeed we never look it up internally). Along the way,
tidy up ssl_update_cache to be more readable. The slight behavior change is
that SSL_CTX_add_session's return code no longer controls the external
callback. It's not clear to me what that could have accomplished. (It can only
fail on allocation error. We only call it for new sessions, so the duplicate
case is impossible.)

The one thing of value the internal cache might have provided is managing the
timeout. The SSL_CTX_flush_sessions logic would flip the not_resumable bit and
cause us not to offer expired sessions (modulo SSL_CTX_flush_sessions's delay
and any discrepancies between the two caches). Instead, just check expiration
when deciding whether or not to offer a session.

This way clients that set SSL_SESS_CACHE_CLIENT blindly don't accidentally
consume gobs of memory.

BUG=531194

Change-Id: If97485beab21874f37737edc44df24e61ce23705
Reviewed-on: https://boringssl-review.googlesource.com/6321
Reviewed-by: Adam Langley <alangley@gmail.com>