client reconfigured as server is still registered in Consul as client

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When the nomad client is started, it will register with consult. The role is client. However, the subsequent nomad server and client do not distinguish the nomad nodes based on this role, which will lead to confusion. For example, a nomad Start with the client role first, and then start it again as the server role. There are nonstandard operations in this operation. There is no cleaning up of local data. The newly started nomad server expects to join another nomad server cluster. However, the test shows that the original nomad server cluster will still find it and force it to join its member list。

This operation is not standardized, but the problems it brings show that if nomad does not actively change the role in the cluster, then the cluster should not actively modify the role of nodes through other ways. I think this logic is reasonable and has little to do with whether the operation is standard or not

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### Nomad version
Output from `nomad version`
V 0.10.4
### Operating system and Environment details

### Issue

### Reproduction steps

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> Subject: GH-1234: Errors garbage collecting allocs

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### Nomad Server logs (if appropriate)