Make cluster.Metadata a struct and stop using mocks for it

[vytautas-karpavicius]

What changed?
Make cluster.Metadata a struct, drop interface, corresponding mock and all its usages.

Why?
cluster.Metadata is a pure state struct that is constructed once when server starts and never change. Thus it is safe to pass it by value, no need for pointers or interfaces. Furthermore, keeping it a simple struct significantly simplifies testing.

How did you test it?
Existing tests.

Potential risks

Release notes

Documentation Changes

[coveralls]

Pull Request Test Coverage Report for Build 018113d3-f495-4c9d-9ded-7510e9a4cccd

• 14 of 26 (53.85%) changed or added relevant lines in 3 files are covered.
• 100 unchanged lines in 18 files lost coverage.
• Overall coverage decreased (-0.06%) to 56.876%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
common/persistence/persistence-tests/persistenceTestBase.go	0	2	0.0%
common/resource/resourceTest.go	0	4	0.0%
common/cluster/metadata.go	14	20	70.0%

Files with Coverage Reduction	New Missed Lines	%
common/cluster/metadata.go	2	68.63%
common/membership/hashring.go	2	83.54%
common/persistence/executionManager.go	2	78.2%
common/persistence/historyManager.go	2	66.67%
common/persistence/statsComputer.go	2	96.43%
common/task/weightedRoundRobinTaskScheduler.go	2	89.64%
service/history/handler.go	2	47.73%
service/matching/matcher.go	2	91.46%
service/matching/taskListManager.go	2	73.77%
common/task/fifoTaskScheduler.go	3	84.54%

Totals
Change from base Build 01810671-81ba-482b-ac33-da8b392cee8c:	-0.06%
Covered Lines:	83974
Relevant Lines:	147645

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💛 - Coveralls