Handle new structured log format (p4d 2019.2)

[rcowham]

Handle new structured logs from p4d 2019.2:

#1828890 (Bug #95112, #90346, #67029) **
Structured logging improvements.
Structured logs now have a versioned schema that allow new versions
of existing events to be added. Updated versions of events are
represented by having the server version included after a period in
the event type field. The event types with new versions this release
are:
2.49 - CommandEnd
6.49 - Audit
8.49 - NetworkPerformance
9.49 - DatabasePerformance
The 'serverlog.version.N' configurable can be used to pin a
structured log file to a specific server version's format.
To retain the prior structured log events format, set the
'serverlog.version.N' configurable to 48.