update-reapply 4: Functional tests of scenarios involving updates and history replication

[dandavison]

This PR adds functional test coverage for various scenarios involving updates and history event replication:

TestConflictResolutionReappliesUpdates: this test passes without requiring further changes (the necessary work was done in the previous PRs addressing WorkflowReset and reapplying updates).

TestAcceptedUpdateCanBeCompletedAfterFailoverAndFailback, TestUpdateCompletedAfterFailoverCannotBeCompletedAgainAfterFailback: to make these tests pass, this PR modifies update registry mechanics: we now update the registry from mutable state every time we obtain the registry from the workflow context. This is needed because, when we failback to cluster 1 after a failover, cluster 1 already has a populated registry, which must be brought up to date with events in history received from cluster2 (and hence in mutable state).

@alexshtin and I are discussing an alternative solution based on dropping the registry instead of updating it, so a future PR may change the way that the tests introduced here are satisfied.

TestConflictResolutionReappliesUpdatesSameIds: this test fails and is skipped; it will be addressed in the next PR.

Potential risks

If the update registry changes here are incorrect then update could be broken in various ways.

Is hotfix candidate?

No

[dandavison]

I'm looking into the test failures, but feel free to review.

[yycptt]

Tests look awesome!