Stack safety of combineRequests

[abestel]

Context
Our application crashed because of a StackOverflowError in combineRequests.
We noticed it because we run the application with -Xss256k.

Proposal
Trampoline combineRequests and make it stack safe.

Side note
During my tests, I noticed that a big batch (5000 elements) was extremely slow to be processed.
Most of the time is spent into combineIdentities (half in NonEmptyList.exists and the other half in NonEmptyList.:+).
By changing FetchQuery to return identities as a Set[I], the code is much faster (the big batch test went from minutes to seconds).
Any objection?

[purrgrammer]

During my tests, I noticed that a big batch (5000 elements) was extremely slow to be processed.
Most of the time is spent into combineIdentities (half in NonEmptyList.exists and the other half in NonEmptyList.:+).

NonEmptyList is not the right data structure to use internally, I was aware this could be a problem so thanks for actually fixing it!

By changing FetchQuery to return identities as a Set[I], the code is much faster (the big batch test went from minutes to seconds).

🚀

The changes look good, I'll merge and cut a new release. Thanks a lot for your time!