[lib] Rename reduceThreadUpdates => generateOpsForThreadUpdates

Summary:
More accurately reflects the behavior of the function

Depends on D4197

Test Plan: No other occurences of "reduceThreadUpdates" in the codebase

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4198