Pass only applicable settings to kv

Currently ns_server passes pretty much all settings to kv (see the function "params(membase," in memcached_bucket_config.erl). These include ones that are not applicable to the type of bucket being created. As an example, backend is set even for ephemeral buckets

{"backend", [], ns_bucket:node_kv_backend_type(BucketConfig)}, 

and

node_kv_backend_type(BucketConfig) ->

    StorageMode = node_storage_mode(BucketConfig),

    case StorageMode of

        couchstore -> couchdb;

        magma -> magma;

        %% KV requires a value but only accepts: couchdb, magma, rocksdb.

        %% So we've always passed couchdb for ephemeral buckets which KV

        %% will parse as an acceptable value but not use it.

        ephemeral -> couchdb;

        %% No storage for memcached buckets

        undefined -> undefined

    end.

It might be nice to specify only applicable settings to kv. MB-61601 is open against kv to do more validation.

Complicating this proposal is the "ensure bucket" that ns_server does. Periodically ns_server asks kv for the settings for a bucket. And if those settings don't match what ns_server has for those settings then the are set again.