res/res_rtp_asterisk: ensure marker bit is correctly set on ssrc change

Certain race conditions between changing bridge types and DTMF can
cause the current FLAG_NEED_MARKER_BIT to send the marker bit before
the actual first packet of native bridging.

This logic keeps track of the ssrc the bridge is currently sending
and will correctly ensure the marker bit is set if SSRC as changed
from the previous sent packet.

ASTERISK-27845

Change-Id: I01858bd0235f1e5e629e20de71b422b16f55759b