CA context not set during disconnect callback

In thinking about https://github.com/pyepics/pyepics/issues/171 I did a quick test of adding prints to the (dis)connection and event callbacks in camonitor.c and find that the thread making the disconnect callback does not (always?) have a CA context associated with it. This means that a disconnect callback which uses functions which implicitly pick up the CA context (like ca_pend_io() ) can implicitly create (and leak) a new context each time.

In my test I find that the disconnect callback is happening on a TCP send thread.