PROTON-1766: [c-epoll] Fix initialization order race
PROTON-1766: [c-epoll] Fix initialization order race

Set the pn_connection_t backpointer *after* the pconnection has been set up.

Otherwise there is a race with pn_connection_wake() on connections that are
re-used after pn_proactor_release_connection()

ChugR/qpid-proton,alanconway/qpid-proton,alanconway/qpid-proton,kgiusti/qpid-proton,alanconway/qpid-proton,ssorj/qpid-proton,ssorj/qpid-proton,kgiusti/qpid-proton,gemmellr/qpid-proton,alanconway/qpid-proton,kgiusti/qpid-proton,apache/qpid-proton,apache/qpid-proton,alanconway/qpid-proton,ChugR/qpid-proton,gemmellr/qpid-proton,ssorj/qpid-proton,astitcher/qpid-proton,ChugR/qpid-proton,astitcher/qpid-proton,ChugR/qpid-proton,ssorj/qpid-proton,ChugR/qpid-proton,gemmellr/qpid-proton,astitcher/qpid-proton,kgiusti/qpid-proton,kgiusti/qpid-proton,astitcher/qpid-proton,alanconway/qpid-proton,astitcher/qpid-proton,gemmellr/qpid-proton,apache/qpid-proton,astitcher/qpid-proton,apache/qpid-proton,ChugR/qpid-proton,ssorj/qpid-proton,gemmellr/qpid-proton,apache/qpid-proton,apache/qpid-proton,gemmellr/qpid-proton,ssorj/qpid-proton,kgiusti/qpid-proton