[lib] Update messageStore.messages with msgStoreOps for createLocalMessage

Summary:
Instead of using messageStore.messages from the previous updatedMessageStore object... we pull it from the processedMessageStore object.

We actually remove updatedMessageStore altogether and include the changes to updatedMessageStore.threads in the messageStore of the returned object.

Depends on D4203

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

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4204