PYTHON-4227 Unified tests: Advance cluster_time of ClientSessions after initialData creation

[caseyclements]

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Changes

After initialData is added in test.unified_format's run_scenario, we get the $clusterTime from the server, and add this to each of the ClientSession entities before further operations.

Motivation for the change.

DRIVERS-2816

Here is the pertinent section of the specifications

[caseyclements]

I'm investigating the new failure that popped up. I can reproduce locally.

FAILURE: pymongo.errors.OperationFailure: PlanExecutor error during aggregation :: caused by :: indexes of target collection db0.coll0 changed during processing., 

self = <test.test_crud_unified.TestUnifiedDbAggregateWriteReadPreference testMethod=test_Database-level_aggregate_with_$out_includes_read_preference_for_5_0+_server>

[blink1073]

That's https://jira.mongodb.org/browse/PYTHON-4356

[caseyclements]

@ShaneHarvey, @blink1073 test.test_transactions_unified.TestUnifiedMongosUnpin.test_unpin_after_TransientTransactionError_error_on_abort is still failing intermittently. This PR had removed the skip. Should I put it back on?

[blink1073]

Should I put it back on?

Yeah, apparently https://jira.mongodb.org/browse/PYTHON-4182 was unrelated, I unblocked it and put it in scheduled.

[blink1073]

Merging so we can check the waterfall afterward.