This changes the existing transient state holder to be thread safe.
Allows for future changes where we need concurrent read and write
operations.

The previous class hierarchy was a bit confusing. With these changes
here, the `DbMessageSubscriptionState` is responsible for all state
changes. When necessary, it reads from and writes to the transient state
directly.

This also adjusts names and package paths to indicate that reading and
writing transient state is not a mutable operation.

The previous class hierarchy was a bit confusing. With these changes
here, the `DbProcessMessageSubscriptionState` is responsible for all
state changes. When necessary, it reads from and writes to the transient
state directly.

This also adjusts names and package paths to indicate that reading and
writing transient state is not a mutable operation.

Although the implementation is equivalent, using `update` instead of
`add` is clearer.