Should fix problem:

Issue a partner call. Then, back self out, which removes active event
from event map. Before backout message reaches other side either:

1) Replies to partner call or
2) Issues its own call back to you.

Previously, in either case, when we receive the partner message on the
initial root, we go into
ReceivePartnerMessageRequestSequenceBlockAction, which tries to find
an ActiveEvent with a matching uuid in our event map and creates one
if one doesn't already exist.

Now, in first case, will silently drop message and in second case,
still creating a new event, but the partner will forward its backout
to us, which will get rid of the event from the active event map.
Apparently, had been forgetting to remove nonatomic active events from active event map.