Results 2 issues of Sidhant Bansal

As discussed in my previous [PR](https://github.com/eclipse/mosquitto/pull/1683) (which I broke accidentally while force-pushing an unintended commit and subsequently closed that PR) Motivation: If you want to make a single publish to...

Uses Z_FULL_FLUSH at the point where flushing is done. Therefore it enables the user to read data between any two flush points. Fixes [1656](https://svn.boost.org/trac10/ticket/1656#no1) for Gzip Filters. More context [here](https://groups.google.com/forum/#!topic/boost-list/0u584AOTIqM)...