Add handling of _minConfirmations reserved parameter

[dcroote]

Closes #1393

• Fetch all requests assuming a minConfirmations of 0 if _minConfirmations reserved parameter set (see comment below)
• Parse _minConfirmations from request
• Reject requests with _minConfirmations larger than BLOCK_COUNT_HISTORY_LIMIT
• Filter requests according to minConfirmations (either from chains[n].minConfirmations or, if allowed and present, _minConfirmations request parameter
• Ignore _minConfirmations passed as a request parameter if _minConfirmations reserved parameter is not present for an endpoint
• Working e2e test
• Refactor such that minConfirmations filtering occurs before API calls are made
• Handle users submitting requests with different _minConfirmations - requests are filtered according to the reserved parameter in the specific request, then those remaining are assigned nonces as before. Per the original spec, the max _minConfirmations for each sponsor is applied to all requests in the queue

[amarthadan]

Looks good, mostly nits. But please try reworking the tests so you don't need to change the startCoordinator interface.
I'm approving to move it along but I will test it later as well.

[amarthadan]

I tested the new feature and it looks like it's working correctly, nicely done 🙂