Ensure process-internal message ID uniqueness

When a storage API command is created internally on a node it is
always assigned a strictly increasing message ID that is guaranteed
to be unique within the process. Some parts of the code use this
as a way to distinguish messages from another. However, uniqueness
(prior to this commit) did not necessarily hold, as the underlying
wire protocol would inherit message IDs _from other nodes_ and override
the generated ID with this. I.e. uniqueness no longer holds.

This had exciting consequences when the stars aligned and a remote
node sent the same ID as one generated at the same time internally
on the receiver node. Luckily, in practice this would only be used
in a potentially ambiguous context when sanity checking shared read
lock sets for the _same bucket_ in the persistence threads. Invariant
checks would detect this is as an attempted duplicate lock acquisition
and abort the process. This has been latent for many, many years,
but we've seen it happen exactly once.

This commit introduces an explicit domain separation between the
node-internal (locally unique) IDs and the ID used by the originator.
The originator ID is maintained and returned over the wire to the
caller when sending a response to the incoming request.

Curiously, we don't actually need this originator ID at all since
the caller maintains explicit state containing the sender command.
Unfortunately we can't simply remove it, since versions prior to
this commit will still use whatever's on the wire.

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