Read all available socket bytes

[dpkp]

This addresses the issue discussed in #1315 by repeatedly reading from a non-blocking socket until there are no available bytes remaining. The current code reads only once per select / poll, which causes excessive poll looping when the broker sends many small packets (see #1315 for more discussion).

[jeffwidman]

Looks good to me.

I noticed you only put one space between code and inline comments. PEP8 specifies two spaces, like this: x = True # comment.