Bugfix: write changes back to *Persistence.DataStores

[Groxx]

Since DataStores is a map[string]DataStore, ds is a copy of the data in the map, and changes are made only to that copy.
So this change writes the (possibly)-changed value back to the map, so it actually takes effect.

I added a test because why not. Prior to this fix it fails with:

--- FAIL: TestConfigFallbacks (0.00s)
    config_test.go:95: 
                Error Trace:    config_test.go:95
                Error:          Should NOT be empty, but was <nil>
                Test:           TestConfigFallbacks
                Messages:       nosql config should contain cassandra config / not be empty

and now it passes.

The NumShards update worked before because it's modifying a *SQL, but I've included an assertion for it to serve as a counter-example / to guard against a bug if it ever becomes a value object as well. Plus I assume we have other "fill in defaults / backfill legacy stuff" values, or we will eventually, and bundling all the simple ones into one test is fairly reasonable.

[coveralls]

Pull Request Test Coverage Report for Build df5f1b18-77de-4e01-abc4-1a1a7bb99248

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 47 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+0.03%) to 60.171%

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Files with Coverage Reduction	New Missed Lines	%
common/persistence/executionManager.go	2	75.5%
common/persistence/statsComputer.go	2	95.71%
service/history/task/fetcher.go	2	77.18%
service/matching/taskListManager.go	2	74.72%
service/history/shard/context.go	9	65.23%
service/history/execution/context.go	14	66.78%
common/persistence/sql/sqlExecutionStore.go	16	59.54%

Totals
Change from base Build 43038319-4203-497d-8795-db6211790382:	0.03%
Covered Lines:	89923
Relevant Lines:	149445

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💛 - Coveralls