Use gRPC outbound by default for internal traffic

[vytautas-karpavicius]

What changed?
Cadence server will use gRPC outbound for internal traffic by default.

User can still opt-out by setting dynamic config system.enableGRPCOutbound to false.
However we should consider TChannel for internal communication deprecated and will work towards removing it in the future.

Why?
Another step in gRPC migration. We have been running gRPC internally for the last few months, it looks stable to be used by default.

How did you test it?

Potential risks

Release notes
Updated changelog.

Documentation Changes

[coveralls]

Pull Request Test Coverage Report for Build 2a90ea16-f28d-465a-8d05-f232a2779aba

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 36 unchanged lines in 7 files lost coverage.
• Overall coverage decreased (-0.008%) to 56.427%

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Files with Coverage Reduction	New Missed Lines	%
common/task/weightedRoundRobinTaskScheduler.go	1	88.6%
client/history/client.go	2	40.74%
client/history/metricClient.go	2	45.94%
service/history/handler.go	2	47.76%
service/frontend/workflowHandler.go	4	58.59%
service/history/historyEngine.go	9	70.24%
common/persistence/sql/sqlExecutionStore.go	16	58.73%

Totals
Change from base Build b8751eb9-3ab1-4d17-b0c7-b3bf73327ab1:	-0.008%
Covered Lines:	79951
Relevant Lines:	141689

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💛 - Coveralls