Changes

do not invoke two Assignments (fixup for re-assignment)

Make sure we wait for Assignment responses before dispatching an
Assignment Request towards RAN.

In particular, when the remote call leg sends its codecs, we may trigger
Re-Assignment to match that codec. Make sure this is done only once:
when receiving another SDP message, wait for the first Assignment.

Implement as an osmo_timer, since there still is no osmo_fsm
implementation for Assignment nor for CC trans FSM. Also it doesn't
belong in the msc_a FSM (it should remain in state COMMUNICATING).

Without this patch, new ttcn3 test TC_lu_and_mo_call_reass_for_mt_codec
sporadically fails, if MNCC with SDP falls in-between Assignment Request
and Assignment Complete.

Related: osmo-msc d767c73a1f93253a54d6a8650a4cf2143353bba0 == I8760feaa8598047369ef8c3ab2673013bac8ac8a
Related: osmo-ttcn3-hacks I402ed0523a2a87b83f29c5577b2c828102005d53
Change-Id: I0f1e1a551aed545b555b9f236fc5967b1e4cc940