Add rate_ctr support to store/retrieve SDR errors through VTY

Introduce a unified implementation-agnostic interface for radioDevice to
signal SDR error counters to upper layers and manage them.
This patch only implements counters for osmo-trx-lms (other devices will
show all counters unchanged during time).

Sample use through VTY:
"""
OsmoTRX> show rate-counters
osmo-trx statistics 0:
   device:rx_underruns:          0 (0/s 0/m 0/h 0/d) Number of Rx underruns
    device:rx_overruns:          0 (0/s 0/m 0/h 0/d) Number of Rx overruns
   device:tx_underruns:          0 (0/s 0/m 0/h 0/d) Number of Tx underruns
 device:rx_drop_events:          4 (0/s 2/m 3/h 0/d) Number of times Rx samples were dropped by HW
device:rx_drop_samples:        513 (0/s 196/m 425/h 0/d) Number of Rx samples dropped by HW
"""

Change-Id: I78b158141697e5714d04db8b9ccc96f31f34f439
8 files changed