Move async file open functionality to transports

This implements async open() in the fstream, stdio and posix transports only, and only for the write/create open mode. @pnorbert , I think this is one of those features that is not exercised in CI at all, and perhaps can't trivially be, lacking a super slow filesystem, so please if there's a way to test other than my own efforts let me know. (I did add a thread sleep into the asynchronous open threads and it still passed tests, so hopefully OK.) @KyleFromKitware , this looks good to TSAN on my end, but a check from you would be appreciated too.

Closes #1990 (closed)