Block on recv instead of loop/polling in MultiSession

When the time comes to wait for responses to arrive, block on the XMPP
socket (which uses select() under the covers) until data arrives,
instead of looping furiously and calling receive in non-blocking mode.
Before this change, waiting on responses resulted in long-running CPU
spikes.

Signed-off-by: Bill Erickson <berick@esilibrary.com>
Signed-off-by: Lebbeous Fogle-Weekley <lebbeous@esilibrary.com>