Bundle of envelopes

[freimair]

Seeing an oszillating pattern of broadcast message numbers in monitoring, @ManfredKarrer and I suspected that the pattern might be caused by the DoS protection mechanisms in place.

This PR introduces a new message that containing a number of existing messages. The hopes are:

• less connection setup overhead on the network level leading to less network load and hopefully better performance
• not to spread the bursts of e.g. RefreshOffer messages over a long time

In case this change has a bad effect on network stability (although we do not believe it is going to) we can simply remove the newly created Capability in a future release and thus, quickly get rid of the feature.

I tested the code extensively by running

• 4 nodes locally
• they knew each other as seed nodes
• one of the nodes was connected to the live bisq network
• had this setup running for days for each iteration

[sqrrm]

Seems like it's a bit risky, but not a bad idea. Need a bit more time to think on it.

[ripcurlx]

Travis already ran through, but it is still showing pending. I'll merge it anyway.
@sqrrm If you find any issues with this implementation I'll revert this changes before the next release.