[Vent] Concurrent consumers to increase throughput

[gregdhill]

Vent is currently limited to a single burrow instance, to improve throughput we need some way to multiplex over multiple remote burrow instances.

Read say 1000 blocks from node 0, then the 1000 blocks after those from node 1 and so on. You could have N goroutines, 1 for each node and then they could race each other to update a offset uint64 using sync/atomic to the current highest block not scheduled, so node 0 would atomically set offset to 1000, then whichever node is ready first (one of the others) would race to update it to 2000, and so on - then they would all stream into buffered chans in a priority queue. = @silasdavis

[gregdhill]

This can also be extended to the burrow dump client. (see also: #1142)