Refactored messenger's I/O in such a way that it can be driven by an
external poll/epoll/select loop as described in PROTON-525. This
involved factoring a bunch of utility I/O code out of the driver and
into a more basic I/O interface (io.h and selector.h), as well as
introducing the pn_selectable_t interface.


git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1572744 13f79535-47bb-0310-9956-ffa450edef68