Add ability to replicate Index definitions via XDCR

While this may not fit every use-case, there are is a certain class of use-case where you want to create a mirror of your main cluster for hot standby purposes.

I.e. if your primary cluster goes down, you can immediately switch all applications to talk to the standby cluster instead.

This works well currently with XDCR for data, however in order for many applications to work correctly when switching, you also need to ensure that you have matching indexes.
This is a lot of overhead for the administrator to ensure that two clusters are manually synchronized in terms of GSI, FTS, Analytics indexes etc.

To ease the burden of administrating the 'hot standby' use-case, it would be great if we could (optionally) replicate the definitions for FTS, GSI indexes or whatever else is appropriate.

Replicating users would also be useful but understand the difficulties there as we do not actually store any passwords etc. These are also usually fewer in number so manually creating a few users for the applications would be more acceptable, especially as there is no 'build' time when creating a new user vs creating an index.

I have marked the component as XDCR as conceptually it's cross datacenter replication-related, but acknowledge that it may not be the most appropriate method in its current form to provide this functionality.