Multiple DCs: Create an option that prevents clients from opening connections across DCs

Some users strictly want to connect locally, so we should create a database option to support this use case. Clients will still have to go across DCs for read versions and commits, so this option will only prevent talking with storage servers across the WAN.

Start by tracing the client and storage-server connection behavior and the existing database options. The change is complete when an option prevents client-to-storage-server connections across the WAN while still allowing cross-DC communication for read versions and commits.

Written by the indexing model from the issue text.