Allow callback on schema import to let upgrade data for channel when schema evolves

[khanaffan]

Channel might like to reorganize it data when schema is evolved.

The actual mechanism need to be sorted out. But it may require two briefcases so app can do lazy conversion.

Another alternative would be to read before data into memory and apply changes to briefcase after schema is imported. But that will work if transformation require involve small set of data.

But generally, if it major changes to channel we might want to take schema lock or if its minor we might stick with locking.

1. Before schema event that notify app
   • In this event we can choose strategy
     • Clone briefcase so full read access to before state of the briefcase.
     • Do not clone instead just read before data into memory if needed.
2. Schema is imported.
3. After schema import changes are applied to channel which would include moving data around.
4. Finalize changes and push changeset out & release locks.