Storage Backend Evolution Tracking

This story is for tracking how storage backend is evolving in the Couchbase Server API and how we manage it in operator.

Currently here is our process:

bucket.spec.storageBackend can be "", "magma", or "couchstore"

the DAC will reject mutations to a bucket resource with the exception of "" -> "couchstore" (no value to server default value)

Operator will ignore storage backend on updates, as this field is immutable on the server API. 

Operator drops storageBackend from requests and responses on server versions <7.0.0

Operator uses storageBackend on responses, and defaults to couchstore on create requests to server versions between 7.0-7.1.0

Operator uses storageBackend on requests and responses on server versions 7.1.0+

Potential changes coming in the future that would require an update:

   - The default of couchstore is changed in a future version of server

   - The immutability of storageBackend is relaxed on a future version of server

This story is for tracking what is changing and when so that we can adjust appropriately.