Add a timeout parameter for controlling how long synapse will wait
for responses from remote servers. For servers that fail include how
they failed to make it easier to debug.
Fetch keys from different servers in parallel rather than in series.
Set the default timeout to 10s.
In the situation where all of a user's devices get deleted, we want to
indicate this to a client, so we want to return an empty dictionary, rather
than nothing at all.
