More stats, knobs and tunings

[petar]

[x] prometheus metric for how long it takes to send messages [x] option for maximum outstanding bytes per peer [x] tighter send timeouts

• Minimum timeout of 10s.
• We add 2s due to latencies.
• Minimum bandwidth of 100kbit/s.
• Maximum message send time of 2M (way more time than necessary). [x] option for number of task workers [x] option for number of engine task workers

Depends on: https://github.com/ipfs/go-peertaskqueue/pull/10/commits Part of https://github.com/ipfs/go-ipfs/issues/8233

[petar]

@aschmahmann this is ready for review. passes on each OS once. the failing tests are all documented flaky.

[petar]

@aschmahmann ready for another pass.