remote_cluster_info module should return remote memcached access info

In 2.1, XDCR will provide an option to replicate to remote memcached instead of CAPI.

This requires XDCR core infrastructure to understand how to access memcached at remote cluster.

For example, in order to replicate remote bucket "default", vbucket 123, XDCR need to know

1) node (ip address) for this vbucket at remote cluster;
2) memcached port of the node where the vbucket lives;
3) credentials to access the memcached at that node

Module remote_cluster_info is a natural home for such information.

Today 1) has been already encoded in the vbucketmap maintained by remote_cluster_info, but 2) and 3) are not available. This task will expand remote_cluster_info module to include 2) and 3).

In particular, XDCR need an API to return 1), 2), 3) above, e.g.,

remote_cluster_info:fetch_remote_memcached_info(RemoteClusterId, Bucket, VBucket)

returns