Changes

Create transaction for all call indepenant SS messages

When an initial call-independent supplementary service message is
received, it is checked whether the message is a REGISTER message.
If it is not, the MSC will reject it by sending a RELEASE COMPLETE
message.

This patch ensures that a transaction is created even if the message
is not a REGISTER message. If the message is not a REGISTER message,
the transaction is freed after sending the RELEASE COMPLETE message.
The release of the transaction immediately releases the BSSMAP
connection, if there are no other ongoing transactions.

Without this patch, the msc_a_fsm would wait 5 seconds for an initial
transaction before releasing the BSSMAP connection.

Related: OS#6427
Change-Id: Ic6765e5d480735e67d97f0f560da24653b26d487