MINOR: log: add a new field "%lc" to implement a per-frontend log counter

MINOR: log: add a new field "%lc" to implement a per-frontend log counter

Sometimes it would be convenient to have a log counter so that from a log
server we know whether some logs were lost or not. The frontend's log counter
serves exactly this purpose. It's incremented each time a traffic log is
produced. If a log is disabled using "http-request set-log-level silent",
the counter will not be incremented. However, admin logs are not accounted
for. Also, if logs are filtered out before being sent to the server because
of a minimum level set on the log line, the counter will be increased anyway.

The counter is 32-bit, so it will wrap, but that's not an issue considering
that 4 billion logs are rarely in the same file, let alone close to each
other.