The channel participation "Remove" does not clear the old etcdraft WAL & snapshots directories

The channel participation "Remove" removes the ledger but does not clear the old etcdraft WAL & snapshots directories that were used by the consenter.

For example, starting a node with raft-ID=1, removing it, and joining it again, this time with raft-ID=4, on a WAL/snapshot created in the past by ID=1 may create problems. This may work, but is highly unrecommended, as a new raft node with a new ID should start fresh without a WAL.

Another example, creating a 3-node channel "mychannel" on o1, o2, o3. Removing the channel completely from all, creating a 1-node channel with the same name on o1. The old WAL is completely incompatible as it has a 3-node membership from a past incarnation of the channel.

As this is relevant to other consensus types as well, the cleanup should abstracted such that every type would need to implement its own cleanup.

See #3992