Increase synchronization between producer/applier threads and stepdown/stepup

SERVER-26970 isMaster can return isMaster: true while in drain mode

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• Closed

SERVER-24536 nodes can run an election while stepping down

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• Closed

SERVER-26403 Clean primary states on stepdown

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• Closed

SERVER-28181 Deadlock involving the mutexes of oplog fetcher and replication coordinator

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• Closed

SERVER-28272 extend timeout in step_down_during_draining.js when waiting for new primary

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• Closed