[lib] Update messageStore.messages with msgStoreOps for processServerRequests action

Summary:
Instead of getting messageStore.messages from the messageStoreAfterReassignment returned by reassignMessagesToRealizedThreads(...)... we use the messageStoreOps returned by reassignMessagesToRealizedThreads(...) to compute the new messageStore.messages.

Note that a messageStore.messages is still being created within reassignMessagesToRealizedThreads(...) and will be removed in a subsequent diff

Depends on D4202

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/D4203