immutable repairer: populate servers-responding properly

immutable repairer: populate servers-responding properly

If a server did not respond to the pre-repair filecheck, but did respond
to the repair, that server was not correctly added to the
RepairResults.data["servers-responding"] list. (This resulted from a
buggy usage of DictOfSets.union() in filenode.py).

In addition, servers to which filecheck queries were sent, but did not
respond, were incorrectly added to the servers-responding list
anyawys. (This resulted from code in the checker.py not paying attention
to the 'responded' flag).

The first bug was neatly masked by the second: it's pretty rare to have
a server suddenly start responding in the one-second window between a
filecheck and a subsequent repair, and if the server was around for the
filecheck, you'd never notice the problem. I only spotted the smelly
code while I was changing it for IServer cleanup purposes.

I added coverage to test_repairer.py for this. Trying to get that test
to fail before fixing the first bug is what led me to discover the
second bug. I also had to update test_corrupt_file_verno, since it was
incorrectly asserting that 10 servers responded, when in fact one of
them throws an error (but the second bug was causing it to be reported
anyways).