Support generic cross-DC replication plans

[etschannen]

We need to change DatabaseConfiguration to allow specifying many more cross-DC replication plans. Some DCs may want synchronous replication, while others that are farther away may want async replication.

Affected code:

• Data distribution needs to track each DC separately. Currently it only tracks up to two DCs based on the remote TLog replication value.
• New epoch needs to process the new DatabaseConfiguration and start appropriate log sets.
• Multiple async replicas for different DCs requires a lot of work handling log router tags.
• Consider cross logSet anti-quorums so you could wait for mutations to be durable in 2 out of 3 DCs.
• Cluster controller log recruitment and better master exists needs to change
• How to specify and change the configuration from fdbcli (or a new tool)

[etschannen]

moving to milestone 6.1, since the JSON changes were all that was needed for 6.0

[etschannen]

Synchronous replication should probably be implemented as satellite and remote tlogs in the same datacenter. We still want to use log router tags, because it will make tlog spilling much more efficient when the datacenter is down.