Merging same JOINs together & introduction of independent JOINs

[hrach]

Merged as part of #545:

• rework DbalJoinEntry
• merge join conditions targeting the same table (alias)
• remove the same on-expression duplicates
• defer JOINs materialization to SQL in DbalCollection
• rework repeated findBy() calls to reuse ICollection::AND infrastructure to utilize JOINs merging

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Merged as part of #546:

• unify groupBy handling via DbalJoinEntry
• refactor aggregators to instance passing instead of custom collection function handling (rework aggregators to single implementation)
• refactor Array to properly implement aggregation on *Junction level
• fix Array handling dependent aggregated conditions
• move/add table alias handling for IAggregator
• allow passing custom table aggregation logic

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This PR:

• wire IAggregation key to table aliasing logic in Dbal

Closes #541
Closes #526