Change how InstanceManager manages instance caching

[santriseus]

This PR changes how InstanceManager creates, caches and disposes of datasource instances.

Fixes #248

Special notes for your reviewer:
Changes:

1. I added tests which could clarify the issues reported in bug. Without the changes from this PR this tests should fail.
2. I put the Dispose and code for checking the value existence in the lock section. To optimize it (99.999...% of calls will be just getting the value from cache and returning to the caller after config check) I implemented the double-check locking.
3. I have replaced simple sync.RWMutex with implementation of named RWMutex, which allow to process instances with different id in parallel without locking.
4. Since I replaced RWMutex with Named RWMutex we starting to have a situation with parallel read/write call to map with cached instances (for example read instance id#1 and write instance id#2), so I had to replace map with sync.Map

[marefr]

Thanks. Codewise looks good. Added some minor comments/suggestions. Still need to test this out manually as well.

[marefr]

See comment

[marefr]

LGTM

[marefr]

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