Bounded keepalive threads

[wylswz]

KeepAlive currently extends Thread, which are relatively expensive to create and maintain.

Is it possible to manage all keepalives using one or just a few threads?

Originally posted by @wylswz in #985

I can try to implement one, and if that works well, I'll start a pull request. Let me create an issue from this.

Originally posted by @wylswz in #985 (reply in thread)

[wylswz]

I'm planning to add a KeepAliveProvider implementation, that produces something equivalent to net.schmizz.keepalive.KeepAliveRunner. Instead of creating a new thread on start, it registers the KeepAlive object to a priority queue managed by configurable number of worker threads.