Update DSL query to allow filtering by missing start time

[neil-xie]

What changed?
Update DSL query to allow filtering by missing start time so we can get count of uninitialized workflow executions records.

Why?

How did you test it?
Unit test

Potential risks

Release notes

Documentation Changes

[coveralls]

Pull Request Test Coverage Report for Build 01848272-0583-4b03-bbda-1571ff909edb

• 8 of 8 (100.0%) changed or added relevant lines in 1 file are covered.
• 100 unchanged lines in 11 files lost coverage.
• Overall coverage increased (+0.03%) to 57.346%

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Files with Coverage Reduction	New Missed Lines	%
common/task/weightedRoundRobinTaskScheduler.go	2	89.64%
service/history/task/transfer_standby_task_executor.go	2	86.4%
service/matching/matcher.go	2	91.46%
service/history/task/fetcher.go	3	91.75%
common/persistence/executionManager.go	4	78.83%
common/persistence/statsComputer.go	4	95.71%
common/task/fifoTaskScheduler.go	5	82.47%
service/history/execution/mutable_state_task_refresher.go	7	62.66%
service/history/shard/context.go	18	66.7%
common/persistence/sql/sqlExecutionStore.go	21	63.86%

Totals
Change from base Build 01843059-58ec-4be4-8188-649341da3156:	0.03%
Covered Lines:	85279
Relevant Lines:	148709

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💛 - Coveralls