steam: added a configurable timeout between poll requests

The Steam servers seem to return empty HTTP replies somewhat frequently,
even during the retries that the SteamHttp interface makes. This issue
is most noticeable when a conversation is in progress, and a lot of poll
requests are being sent. This issue typically makes itself known via
poll requests. In fact, I have yet to see it happen on any other type of
request.

There are many variables that come into play with this issue, but it
only seems to occur with poll requests when a lot of them are being
sent. This brings to mind some type of throttling that is being used on
the Steam servers to prevent excessive polling. A "simple" fix to this
issue is to not send poll requests one right after another, but instead,
introduce a timeout in between each poll request.

This patch introduces a configurable timeout that is used in between
each poll request. However, if there is some sort of throttle for poll
requests, it is also possible that the throttle extends to all requests.
At this point, the issue is only being seen with poll requests, so this
patch only takes that into account. If a global throttle is needed on
the plugin's end, that will be implemented in a different patch.

Since this patch adds in a timeout between poll requests, the latency
of message retrievals will be higher. While the latency of message
retrieval is already somewhat high due to the HTTP base, the timeout
should not that noticeable.

The timeout is configurable via a new set option, `poll_timeout`. The
timeout is specified in milliseconds. If `poll_timeout` is set to 0,
the timeout between requests is completely disabled, returning to the
old functionality.