Clustering: Remove the member role name from the shard's persistent ID

Currently we prepend the shard's persistent ID with the member role name. This presents a couple drawbacks:

1) you can’t change the member name or the shard data wont be recovered from persistence on restart.

2) you can’t backup a journal on one node and restore it on another node.

We should omit the member role name and/or allow for the persistence ID to be specified in the module-shards file.

To preserve data for existing installations we'll need to migrate the existing persisted shard data to the new persistent ID.

Note: we need to keep the member role name as part of the shard name as we use that to identify the raft peers.