SERVER-11332 Maintain a blacklist of unreachable config servers.

This patch adds _connBlacklist and related helpers to
SyncClusterConnection. If a read query on a config server fails,
the mongos will add it to the blacklist so that further queries
can skip that config server and go straight to the next one.

After _blacklistTimeout we retry a query on the config server to
see if it has come back. (In the scenario of SERVER-11332 this
will again cause the query(ies) to block while trying. A future
patch could add a different method of checking when the config
server returns, so that no user generated queries will block
unnecessarily.)

Write operations are unaffected and work as before.

Caveats:
 - NOT tested with our own test suite becuase I don't know what
   to do. The patch doesn't change results or behavior, only
   makes a certain case faster, so all tests should pass unchanged.
 - Does NOT add regression tests for the bug it fixes, I don't
   know how.
 - NOT reviewed.