Shove CowOperation type into source_info

We can shove type into source info to save 8 bits in per cow operation.
We only need 4 bits inside of source_info to enumerate all the types of
Cow Operation. Since CowOperationV3 is not used on disk(just yet) , we
can make format changes.

This CL is mechanical:
    1. Remove tye .type field from CowOperation struct
    2. Add a type() getter method to CowOperation struct
    3. Replace all existing usage of `type` member with the new getter

No functional changes, just refactorings.

Test: th
Bug: 304602386
Bug: 313962438

Change-Id: I85d89c71fc6afede12ea299a4a3e3b2184ea2d8b