tools/redact: remove sensitive information from member directory

[heyitsanthony]

Sometimes wal logs and boltdb data have sensitive data that can't be shared. This is bad for debugging since the metadata is useful for finding why a cluster has broken.

To get that metadata, etcd needs a tool that will redact keys/values/cluster info from a member directory while preserving everything else (raft indexes, etcd revisions, etc).

[xiang90]

/cc @jpbetz

[xiang90]

@jpbetz do you want to work on this for the next release cycle?

[stale]

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