[lib] Replace mergedStore.messages with processedStore.messages in mergeNewMessages

Summary:
This diff removes the second-to-last usage of assertMessageStoreMessagesAreEqual(...) in message-reducer. The last usage is also in mergeNewMessages, but the logic there is more involved so I'm going to take a second to read through it again and put up a seperate diff.

Depends on D4331

Test Plan: This should be a straightforward refactor given we've had assertMessageStoreMessagesAreEqual in place for some time now.

Reviewers: def-au1t, palys-swm, yayabosh

Reviewed By: palys-swm, yayabosh

Subscribers: ashoat, Adrian

Differential Revision: https://phab.comm.dev/D4332