[Format] Incremental query execution

[lidavidm]

Flight RPC/Flight SQL is proposing a way to execute queries and get back results incrementally, instead of waiting for the entire query to finish: https://lists.apache.org/thread/247z3t06mf132nocngc1jkp3oqglz7jp

We should reflect that capability here.

Proposal

Add a new option that can be enabled on statements. When enabled, ExecutePartitions will return whenever new partition(s) are available, instead of blocking and returning only when the query is finished. (Query progress/cancellation are handled by #458.) If no partitions are returned, then the query is finished.