Avoid disconnecting all peers if user code is slow

Avoid disconnecting all peers if user code is slow

In the sample client (and likely other downstream users), event
processing may block on slow operations (e.g. Bitcoin Core RPCs)
and ChannelManager persistence may take some time. This should be
fine, except that we consider this a case of possible backgrounding
and disconnect all of our peers when it happens.

Instead, we here avoid considering event processing time in the
time between PeerManager events.