Major Description
Currently, RowDataSourceScanExec and FileSourceScanExec rely on a "metadata" string map to implement equality comparison, since the RDDs they depend on cannot be directly compared. This has resulted in a number of correctness bugs around exchange reuse, e.g. SPARK-17673 and SPARK-16818.
To make these comparisons less brittle, we should refactor these classes to compare constructor parameters directly instead of relying on the metadata map.
