Sanity check on ledger storage type

**FEATURE REQUEST**

1. Please describe the feature you are requesting.

Right now there's nothing to stop you from changing the ledger storage configured in bk_server.conf and booting up, even if you had another ledger storage configured before, and data for a bunch of ledgers stored there.

If you change ledger storage like this, you will silently lose data. The bookie will appear to be available, and the system will only notice the ledgers are missing when it tries to read them.

2. Indicate the importance of this issue to you (blocker, must-have, should-have, nice-to-have). Are you currently using any workarounds to address this issue?

should-have.

3. Provide any additional detail on your proposed use case for this feature.

We should add the ledger storage to the cookie. If the ledger storage changes, it should require a format of the bookie.