IuCS: rapidly release connections

Do the same as we do in 2G: release the connection as soon as nothing else
is pending for a given subscriber.

Before, osmo-msc would wait for the UE "to get bored" and send an Iu
release. But the CN should stay lean on connections. Also, 25.413[1] in section
7, 6th point states:
"While the Iu release is managed from the CN, the RNC has the capability to
request the release of all Iu connection resources from the corresponding Iu
connection."
So far we did not manage Iu release from osmo-msc at all.

Use the same mechanism we use in 2G: from msc_release_connection(), just before
freeing the gsm_subscriber_conn, invoke a CN initiated Iu Release command to
the UE.

This works around OS#1816 ("USSD only works when IuCS is released", on nano3G),
because the Iu conn is now released right after every signalling, so that
typically no two requests will use the same conn.

In iu.h/iu.c, add iu_tx_release(), absorbing almost all of the code from
ranap_handle_co_iu_rel_req().

Add stub to db_test.c, necessary to build it without linking libiu.

[1] 3GPP TS 25.413 v12.4.0 Release 12 / ETSI TS 125 413 V12.4.0 (2015-04)

Related: OS#1816
Change-Id: Ic12bd6f3666f6fd42bd6d9fdae1c93abee3b6786

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