added default hwm and per-subscriber queues

[wyang5]

Hey! I'm a fan of this library. A couple of tweaks:

• Default high-water mark:
  • The HWM is currently set to zero, which allows the queue to grow infinitely - PUB / SUB sockets #151.
  • The spec states that we "SHOULD constrain queue sizes to a runtime-configurable limit". In libzmq, the default is 1000, so that seemed like a sensible choice. Side note, there was a TODO to allow for "per subscriber hwm". The zmq docs are a bit vague in some places but it seems like it's supposed to be per-socket rather than per-subscriber.
  • This allows us to use Go's buffered channels instead of Queue.
• Per-subscriber queues:
  • Currently, the pub socket maintains a single outbound queue, which can cause issues (e.g. what happens if a single connection hangs?)
  • Instead, we can spawn a goroutine for each Conn added. This also address this TODO.
  • Again looking at the spec: we "SHALL maintain a single outgoing message queue for each connected subscriber".

[Flixnore]

Would love to see this resolved @sbinet

[sbinet]

LGTM.

please send another PR against go-zeromq/license adding yourself to the AUTHORS and/or CONTRIBUTORS file(s).

thanks.

[sbinet]

needs go-zeromq/license#17

[sbinet]

thanks for the PR and sticking with us.