Change KVS wait to a loop that alternates between checking KVS and file lock

[ilumsden]

Recommended by @hariharan-devarajan

Instead of using WAITCREATE with the Flux KVS, we can do an asynchronous KVS read. Then, we can busy poll. In each iteration of the busy poll, we would (1) check KVS read and (2) check if file has been created.