Added rate-limiting to (most) error logs generated by varnishkafka (issue #1)

Controlled through properties:
 - log.rate.max
 - log.rate.period

Change-Id: Ia535f164bd4a6c088441fe4fbd769f4ce39fa05f
Added statistics (both from varnishkafka and librdkafka)


Configuration property:
  "log.statistics.interval"

Change-Id: Ia427113314b0960036b54825b70173331a58d29a
Grow scratch pad by temporary buffers if necessary (issue #2)

Change-Id: I89319eea4b46eb3a9063a3e5764135e861a46e7f