[lib] Split WebInitialReduxStateResponse into Client/Server variants

Summary:
Split WebInitialReduxStateResponse into a Client variant with +threadStore: ThreadStore and Server variant with +threadStore: LegacyThreadStore to support older clients.

This should be the last diff in the stack.

Depends on D10895

Test Plan: CI + flow + close reading

Reviewers: ashoat, ginsu, tomek

Reviewed By: ginsu

Differential Revision: https://phab.comm.dev/D10898