raft: Replace partition key with a hash as a singular identifier

As discussed in the Slack conversation - https://gitlab.slack.com/archives/C087FQ4MAAG/p1744172584678519, after a partition is created, it has no real attachment to the storage that created the partition ID, so instead we can use a hashed ID as a singular identifier.

Edited Jun 30, 2025 by James Liu

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information