Stream concurrency for strfry router

[braydonf]

I've been reading through the source of src/apps/mesh/cmd_router.cpp and it appears to all run in a single thread with an event loop, I believe this can be an issue when running with hundreds of streams, and would like to add some concurrency to the router.

Reading through other sources such as src/apps/relay/cmd_relay.cpp for strfry relay and multiple ThreadPool instances are used to support concurrency.

Is this a good idea?