Set pub_hwm to 1000 by default, Fix #7211

@thatch45 should probably be the one to merge this.

@olliewalsh pointed out in #7211 that a HWM of 1 means that you can't have any pubs queued on the wire, which is a problem for concurrent CLI clients or salt API.  We obviously don't want no HWM, but it makes sense to default this a little higher than 1, unless I'm missing something.