Writing JSON statistics to log file rather than syslog or stderr

syslog imposes a default limit of 2K on log lines.  Some
of these JSON formatted statistics lines can be larger than
that.  I am seeing lines over 3K.

Rather than try to use syslog for statistics reporting, we now
write directly to a log file.  The lines that are written there
are now fully valid JSON.  Each line may be parsed as its own
JSON object, without any extra non-JSON metadata prepending it.
(syslog prepends facility, timestamp, etc.).

Log rotation is handled via reopening files on SIGHUP.

The logfile defaults to /var/cache/varnishkafka.stats.json, and
is configurable via the log.statistics.file property.

This also adds a 'time' entry for varnishkafka stats.

Change-Id: I501fe6b421325ae98e21f910a7ae367762075371