Avoid missed-request bug when entire http request arrives before data is flushed

Avoid missed-request bug when entire http request arrives before data is flushed

The trigger for starting to read the first line of a request used to
be, "When data has arrived and we're looking for the first line."
But that's not good enough: if the entire next request gets read
into our bufev->inbuf while we're still processing the current
request, we'll never see any more data arrive, and so will never
process it.

So the fix is to make sure that whenever we hit evhttp_send_done, we
call evhttp_read_cb.  We can't call it directly, though, since
evhttp_send_done is reachable from the user API, and evhttp_read_cb
can invoke user functions, and we don't want to force everyone to
have reentrant callbacks.  So, we use a deferred_cb.

Found by Ivan Andropov.  This is bug 3008344.