Move "use_mki" from a per packet parameter to a stream wide value.
When using MKI all master keys need to have a valid MKI id.

Merge the MKI protect & unprotect functions. The MKI index is currently still
passed per packet, this could also be moved up to stream level with a
srtp_set_mki(ssrc) api as this is not likely to change per packet.

RFC 3711 section-3.2.1 states that the use of MKI and the size of
the MKI id must be fixed for the life time of stream. By moving
the mki_size value along with mki_use value to the stream level
allows this to be enforced for all keys.
Added a unit test to verify it can not be changed via an update.