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This patch introduces an adaptive dial queue that spawns a dynamically sized
set of goroutines to preemptively stage dials for later handoff to the DHT
protocol for RPC. It identifies backpressure on both ends (dial consumers and
dial producers), and takes compensating action by adjusting the worker pool.

We start with `DialQueueMinParallelism` number of workers (6), and scale up
and down based on demand and supply of dialled peers.

The following events trigger scaling:
- we scale up when we can't immediately return a successful dial to a new
  consumer.
- we scale down when we've been idle for a while waiting for new dial
  attempts.
- we scale down when we complete a dial and realise nobody was waiting for it.

Dialler throttling (e.g. FD limit exceeded) is a concern, as we can easily
spin up more workers to compensate, and end up adding fuel to the fire. Since
we have no deterministic way to detect this for now, we hard-limit concurrency
to `DialQueueMaxParallelism` (20).

Revert "Remove signal bootstrapping"

Remove signal bootstrapping

Remove IpfsDHT.BootstrapOnSignal.

* add RoutingTable func

* fix gofmt

Both, dht.go and dht_bootstrap.go duplicate the package documentation.
The whole block had been moved to doc.go

Also query.go provides documentation for query package but now it
belongs to dht package, so it's been removed.

fix a potential DHT query hang

gx publish 4.4.19

Without this, one of the workers may hang when trying to re-grab the rate-limit
after dialing. I believe this may be what's causing some of our DHT requests to
stall (evidence: massive goroutine buildup on the gateways).

docs: duplicate pkg documentation

Also contained in "dht.go" and prints twice in godoc.

gx publish 4.4.18

tests: skip key inlining test

This has temporarily been disabled due to the changes/issues described in:

https://github.com/libp2p/specs/issues/111

Rephrase "betterPeersToQuery" method comment to be less cryptic

gx publish 4.4.16