Deprecate subscription_queue mechanism, handle non memory comparable keys via checksum

[morgo]

Currently if a key is non-memory comparable, we force the replication subscription to be a queue, and not the delta map.

This is considerably worse for performance for what is an obscure case (the character order does not match the binary comparable order).

What we could do instead is:

• Enforce that the checksum must be enabled for such cases.
• Fix any inconsistencies detected in the checksum phase.

[morgo]

There's one problem I don't know a good answer for:

• Yes we can correct issues with the checksum
• But after the checksum runs there is a race where the last few modifications are not technically checksummed.

One possible solution is to switch back to a queue after the checksum, but I'm not sure that's very elegant.