Also adds crate-private module for handling schema differences

This change fixes an issue when using a JetStream K/V store where a user
is creating, deleting, and re-creating keys. If the last entry for a key
is a `Operation::Delete` or `Operation::Purge`, the initial
`self.update()` returns an error, causing the second part of the method
to be exercised.

Prior to this change, if the entry was deleted or purged a `kv.put()`
call is used which ignores the revision of that last entry. A single
writer to the K/V store would succeed (as no other writers would write
first) so no problem. However, if 2 writers attempt to create a key,
then a second writer *could* call the `kv.put()` before the first writer
calls `kv.put()`. This means that *both* writers get an `Ok(revision)`
and can assume that they won the creation of the key.

When using a "distributed lock" pattern (that is many writers race to
create a key and the first successful writer wins), this above scenario
results in potentially more than one writer who believes they have
uniquely acquired the distributed lock.

This change replaces the `kv.put()` call to a `kv.update()` call and
provides the `revision` from the deleted/purged entry to ensure that no
other writer has beaten the caller to this update. This change closes
the race period between concurrent writers to between the first update
and the second update call with some optimistic write concurrency to
detect another writer.

It appears as though this strategy is in effect in the Go client code
[kv.Create] implementation.

[kv.Create]: https://github.com/nats-io/nats.go/blob/278f9f188bca4d7bdee283a0e98ab66b82530c60/jetstream/kv.go#L944-L963

Co-authored-by: John Keiser <jkeiser@systeminit.com>
Signed-off-by: Fletcher Nichol <fletcher@systeminit.com>
Signed-off-by: Fletcher Nichol <fnichol@nichol.ca>

Also adds private module from handling schema differences

This reverts commit f2b24ca.

This reverts commit 8493766.