HTTP UDFs do not correctly handle results rooted in an array

When the HTTP udfs http_get or http_request are used against an endpoint that returns JSON data that is rooted with an array (or line-delimited object sequence) then they return consecutive objects from that array with each invocation until the end is reached, after which the next invocation fails with an error. This can be reproduced by having a web server serve Drill's donuts.json.

In addition, each invocation of these UDFs leaks a single file descriptor.