[lib] Introduce deprecatedConvertClientDBThreadInfoToRawThreadInfo

Summary:
Introduce deprecatedConvertClientDBThreadInfoToRawThreadInfo which turns a ClientDB representation of RawThreadInfo into LegacyRawThreadInfo.

The reason for this function, as described in D10198, is to maintain correctness of existing Redux migrations.

(As an aside, Redux migrations would be a great thing to write unit tests for because often our assumptions about "current state of the codebase" when modifying helper functions don't apply to migrations and can break them. I just happened to notice convertClientDBThreadInfoToRawThreadInfo when looking at creating a new Redux migration and realized this would break old migrations.)

Depends on D10209

Test Plan:
NA/CI/flow

Reviewers: ashoat, ginsu, tomek, rohan

Reviewed By: rohan

Differential Revision: https://phab.comm.dev/D10210