Combine BatchPoints with the same RoutingTag to one message in amqp output

[ekini]

After some testing, I've found that 80 hosts can generate up to 11k messages per second and send it to rabbitmq, which is quite a lot!
So let's do some batching.

Also, according to https://www.rabbitmq.com/blog/2012/04/25/rabbitmq-performance-measurements-part-2/

Quite a lot of the work done by RabbitMQ is per-message, not per-byte-of-message.

Which seems logical.

So this commit uses []byte to accumulate output. It also honours RoutingTag.

Just a few numbers:
80 hosts, recent telegraf, only system metrics.
Before: 11k msg/s (peaked), LA on influxdb host >3 (4 processors)
After: 3k msg/s, LA on influxdb host ~ 0.15

[sparrc]

@ekini I've made some substantial changes to the accumulator and how output sinks are handled. Could you please rebase or re-write your changes off of HEAD?

sorry about that

[ekini]

Ok, updated.

[sparrc]

great, thanks, will get this merged 👍