Use libvlr in libmsc (large refactoring)

Original libvlr code is by Harald Welte <laforge@gnumonks.org>,
polished and tweaked by Neels Hofmeyr <nhofmeyr@sysmocom.de>.

This is a long series of development collapsed in one patch. The original
history may still be available as branch neels/vlr_orig.

TODO: This commit may be split in several smaller changes before merging to
master.

SMS:

The SQL based lookup of SMS for attached subscribers no longer works since the
SQL database no longer has the subscriber data. Replace with a round-robin on
the SMS recipient MSISDNs paired with a VLR subscriber RAM lookup whether the
subscriber is currently attached.

If there are many SMS for not-attached subscribers in the SMS database, this
will become inefficient: a DB hit returns a pending SMS, the RAM lookup will
reveal that the subscriber is not attached, after which the DB is hit for the
next SMS. It would become more efficient e.g. by having an MSISDN based hash
list for the VLR subscribers and by marking non-attached SMS recipients in the
SMS database so that they can be excluded with the SQL query already.

There is a sanity limit to do at most 100 db hits per attempt to find a pending
SMS. So if there are more than 100 stored SMS waiting for their recipients to
actually attach to the MSC, it may take more than one SMS queue trigger to
deliver SMS for subscribers that are actually attached.

This is not very beautiful, but is merely intended to carry us over to a time
when we have a proper separate SMSC entity.

Introduce gsm_subscriber_connection ref-counting in libmsc.

Related: OS#1592

Change-Id: I702ba504ce2de93507312c28eca8d11f09f4ee8b

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