a_reset: Rename SIGTRAN connection to BSSMAP MSC assocation in log messages

Explanation from Harald:
There are plenty of things that can happen above the bare SCTP/M3UA
connection which can happen, such as SCCP or MTP level routing
problems.

The fact that a MSC responds to a BSSMAP reset tells us that all of
the underlying SS7 transport network is operational and the MSC
responds to us.

Go draw an IP analogy: Saying "SIGTAN connection up" here is like
saying "Ethernet link up" when you get an ICMP response.

Change-Id: If9d37c94f2f2b6cffef97f445774766993f538db
1 file changed