validate uniqueness fixes and recurse into array elements, other improvements

[tomreitz]

This PR:

• fixes a bug which made the uniqueness validation method always succeed (as the uniqueness hash list was emptied before processing each payload)
• improves performance of the uniqueness validation method by moving certain preparatory steps (building the identity parameter structure, resolving the swagger reference definition, etc.) outside the payload processing loop (so it's only done once, not for every payload)
• implements recursive uniqueness validation of array elements within payloads, as requested/discussed in this item
• improves validation error messages to include the validation method that failed