Consolidate CCS_OK codepaths in s3_srvr.c.

Rename SSL3_ST_SR_POST_CLIENT_CERT to SSL3_ST_SR_CHANGE and have this be the
point at which CCS_OK is set. The copy before ssl3_get_finished is redundant as
we never transition to SR_FINISHED directly.

Change-Id: I3eefeb821e7ae53d52dacc587fdc59de9ea9a667
Reviewed-on: https://boringssl-review.googlesource.com/1297
Reviewed-by: Adam Langley <agl@google.com>