introducer.py: accelerate reconnection after being offline. Closes #374.

introducer.py: accelerate reconnection after being offline. Closes #374.

When we establish any new connection, reset the delays on all the other
Reconnectors. This will trigger a new batch of connection attempts. The idea
is to detect when we (the client) have been offline for a while, and to
connect to all servers when we get back online. By accelerating the timers
inside the Reconnectors, we try to avoid spending a long time in a
partially-connected state (which increases the chances of causing problems
with mutable files, by not updating all the shares that we ought to).