osmux: Rotate over available Osmux CID when allocating a new one
osmux: Rotate over available Osmux CID when allocating a new one

Before this patch, the free CID with the smallest number was always
selected to be used. This caused more or less the same subset of CIDs to
be used all the time, while the CIDs with bigger numbers were mostly
unused.
Let's distribute the use so that all CIDs are used roughly the same.
This has the advantage, among others, that the same CID will not be
re-used immediatelly after being freed if a new call is established.
It is useful to leave the CIDs unused for some time since the other end
peer may know of the call being tear down with some delay.
Hence if a new call is established immediately after the CID was
released, the same CID would be allocated and passed at the peer, which
would then detect that the old call (in its view still active) would
already make use of that remote CID.

Related: SYS#6161
Change-Id: I72803fb172accbabfc81923572890f8ecb06cefd

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts