Metadata should become read-only when mongos detects config databases being different

If mongos detects that config servers are different, metadata should not be changed.

• Ongoing split/moveChunks operations can lead to the full inconsistency if the first config is corrupted for some reason;
• Ongoing metadata changes can mask the original issue

I checked manually with 2.4.1, and I could do manual splitAt and moveChunk, while my second config server was out of sync.