P2P, HTTP, SHiP: Change fc::create_listener to use strand

[heifner]

Currently fc::create_listener creates sockets with the provided executor. For our use cases, it would be nice if fc::create_listener would create a strand for each socket. The users of fc::create_listener could then retrieve the strand from the socket.get_executor() for use.

Something like:
438ae62

Then:

• Update SHiP to use that strand via socket.get_executor() instead of making a new strand.
• Update net_plugin to use the socket strand instead of creating a different one.

[spoonincode]

create_listener shouldn't blanket do this because some users of it (http) don't actually need the per connection strand. It should be driven by a parameter or different function name

[spoonincode]

Actually, because of #816 (comment) I'm not sure create_listener should simply make the strand on the same executor it's using for listening. For this particular case it really seems best for the listener to remain on the main thread and have the socket with a strand in ship's thread. So maybe create_listener needs to take a functor that is called on each async_accept() that returns the executor for that call. To provide the most flexibility.