Move handshake method hooks to SSL_PROTOCOL_METHOD.

None of these are version-specific. SSL_PROTOCOL_METHOD's interface will change
later, but this gets us closer to folding away SSL3_ENC_METHOD.

Change-Id: Ib427cdff32d0701a18fe42a52cdbf798f82ba956
Reviewed-on: https://boringssl-review.googlesource.com/3769
Reviewed-by: Adam Langley <agl@google.com>