CSHARP-1378 BulkWrite enumerates requests argument multiple times

[M-frankied]

BulkWrite currently enumerates its requests param 3 times:

• to check whether it is empty
• to build the bulkwrite operation
• to copy requests into the result object

That behaviour can be very costly if requests represents e.g. a remote db table.

This pull requests contains 2 commits:

• CSHARP-1378 enumerate bulkwrite request list once: Simply copies the enumerable to a list once and uses that list everywhere the enumerable was used. Includes a unit test that fails for the case where BulkWrite/BulkWriteAsync enumerates the enumerable multiple times.

• CSHARP-1378 remove extra list copy: With the above commit BulkWrite does one more list copy than in the baseline case. This second commit optimizes that extra copy away because I don't want to worsen performance for cases where requests is cheap to enumerate. I put it in a separate commit because you might find avoiding the extra list copy not worth the hassle because "computers are fast now".

Some tests fail in my ad-hoc environment, but then they also fail there without these changes, so that should be ok.

[sanych-sun]

Thank you for your contribution. This PR was very helpful and excellent starting point for me to fix the original issue in another PR #1298.