Aggregate stage-split operations into WCT

Previously different operations were just applying transactions
as they were called. We need better control over this for
synchronization purposes, so we basically pass wct into these
operations so that everything can be aggregated into one WCT
if desired.

Bug: 192291727
Test: existing tests pass (this is a refactor)
Change-Id: Ie26d634bcd3ab6baf4d2a4788986689914a58c13