implement API for queries via GET (Fixes #350)

Make output path optional when executing queries. If not specified,
the evaluator simply leaves a result temporary collection in place.

Added a TODO on cleaning up these temporary collections.
delete temporary collection after reading results

- remove “requested output” collection param from Evaluator
- Backend now moves the temp collection to the requested location when
necessary (or deletes it)
- REPL and API (GET) both get temps cleaned up for free
- “native” (JavaScript) plan no longer has the destination of the rename
use Process combinators to clean up (and don't rely on laziness of onComplete)
add cleanUpWith to Process
Merge pull request #381 from mossprescott/ready/350

implement API for queries via GET (Fixes #350)