For each conn, set a default logging category, to distinguish categories for
BSSMAP and RANAP based conns.
LOG_RAN_CONN(): log with the conn's default category,
LOG_RAN_CONN_CAT(): log with a manually set category (mostly for keeping
previous DMM logging on the same category).
In some places, replace LOGP() using manual context with LOG_RAN_CONN(), and
remove the manual context info, now provided by the conn->fi->id.
This is loosely related to inter-BSC and inter-MSC handover: to speed up
refactoring, I want to avoid the need for manual logging context and just use
this LOG_RAN_CONN().
Change-Id: I0a7809840428b1e028df6eb683bc5ffcc8df474a
46c06e28c1