Return early when there are no replication tasks

[vytautas-karpavicius]

What changed?

• Return early when reading replication tasks if it is clear that no task will be returned. Return empty response immediately.
• When generating task IDs, ensure that replication tasks will get generated last. This will mean that we can later quickly compare the last shard task ID (with GetTransferMaxReadLevel) with lastReadTaskID coming from remote cluster to decide if there are any potential replication tasks to fetch.

Why?
GetReplicationTasks is one of the most intensive DB operations. It is called periodically when remote clusters fetch replication messages (for each shard). It is entirely possible that a shard did not encounter any activity since last call. We can check that and skip DB call.

How did you test it?

• Unit test
• Tested on staging2. It significantly reduces db calls. Although traffic is low on there. On production it may not be that significant.

Potential risks

Release notes

Documentation Changes

[coveralls]

Pull Request Test Coverage Report for Build 0182f7a2-d4ae-4143-936a-316742c6ba89

• 10 of 12 (83.33%) changed or added relevant lines in 2 files are covered.
• 153 unchanged lines in 19 files lost coverage.
• Overall coverage decreased (-0.05%) to 57.822%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
service/history/shard/context.go	5	7	71.43%

Files with Coverage Reduction	New Missed Lines	%
client/history/client.go	2	38.1%
client/history/metricClient.go	2	45.3%
common/persistence/executionManager.go	2	78.83%
service/history/execution/mutable_state_util.go	2	36.14%
service/history/handler.go	2	47.17%
service/history/task/transfer_active_task_executor.go	2	71.58%
service/history/task/transfer_standby_task_executor.go	2	87.21%
service/matching/taskListManager.go	2	77.11%
service/frontend/workflowHandler.go	4	59.98%
common/persistence/statsComputer.go	5	94.64%

Totals
Change from base Build 0182f5a6-6cfb-4b7b-85a5-434e50ab67c4:	-0.05%
Covered Lines:	84485
Relevant Lines:	146113

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💛 - Coveralls