Adds _servers_with_shares to ServermapUpdater

When calculating the query boundary for updates to mutable files,
instead of using servers that used to have shares, use servers we
have added to the servermap. This way the querying process won't finish
until we have finished interacting with the servers that have shares.

This fixes the race condition which sometimes caused the querying process
to finish before the updater was done talking to servers with shares.

src/allmydata/mutable/servermap.py

@@ -483,6 +483,7 @@ class ServermapUpdater:
         self.full_serverlist = full_serverlist # for use later, immutable
         self.extra_servers = full_serverlist[:] # servers are removed as we use them
         self._good_servers = set() # servers who had some shares
+        self._servers_with_shares = set() #servers that we know have shares now
         self._empty_servers = set() # servers who don't have any shares
         self._bad_servers = set() # servers to whom our queries failed
 
@@ -865,6 +866,7 @@ class ServermapUpdater:
         # Add the info to our servermap.
         timestamp = time.time()
         self._servermap.add_new_share(server, shnum, verinfo, timestamp)
+        self._servers_with_shares.add(server)
 
         return verinfo
 
@@ -1127,7 +1129,7 @@ class ServermapUpdater:
                             found_boundary = True
                             break
 
-                elif server in self._good_servers:
+                elif server in self._servers_with_shares:
                     # yes shares
                     states.append("1")
                     #self.log("loop [%s]: 1" % server.get_name()