riemann: Add batched event sending support

This makes the riemann destination respect flush-lines(), and send event
in batches of configurable amount (defaults to 1). In case of an error,
all messages within the batch will be dropped. 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>