riemann: Add batched event sending support

This makes the riemann destination respect log-fifo-size(), and send
event in batches of configurable amount (defaults to 1). In case of an
error, all messages within the batch will be retried, and eventually
lost if retries do not succeed. Dropped messages, and messages that
result in formatting errors do not count towards the batch size.

There is no timeout, but messages will be flushed upon deinit.

This fixes #284.

Requested-by: Fabien Wernli
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>