Changes

asp: Refactor osmo_ss7_asp_restart() into smaller chunks

This is a preparation patch towards fixing osmo_ss7_asp_restart() doing
too many things at the same time, like tearing down the stream, FSMs,
recreating them etc, not always in the best order.
This function split steps into functions, better ordering the steps in
the process.
For instance, when reopening the stream_cli, make sure to close it
beforehand. This would anyway be done inside osmo_stream_cli_open() but
then all events (and hence logging) even weird, due to disconnect_cb
triggering in the middle of the creation and then calling
osmo_ss7_asp_restart() again while it is being called.
The later will be fixed in a follow-up commit.

Change-Id: I5c3461b724924f528b46f7fe815f017a5b4a1ff5