Count RSL DELETE INDICATION received from BTS

If the BTS downlink CCCH (PCH + AGCH) queue is full, it sends
us an RSL DELETE INDICATION.  So far, osmo-bsc logs this as

<0004> abis_rsl.c:2026 Unimplemented Abis RSL TRX message type 0x14

which is not very helpful.  Instead, make the log message more
descriptive and add a rate counter for monitoring.

Change-Id: I9bd2966db90e39ccca442d6bc9abc91e9a9147d4
Closes: OS#3190
2 files changed