Add "--background" and "--count" flags to "bin/webhook call"

Summary:
See PHI1794, which reports an issue where a large number of queued webhook calls led to connection exhaustion. To make this easier to reproduce and test, add "--count" and "--background" flags to "bin/webhook call".

This primarily supports "bin/webook call ... --background --count 10000" to quickly fill the queue with a bunch of calls.

Test Plan: Ran bin/webhook call in foreground and background modes, with and without counts. Saw appropriate console and queue behavior.

Differential Revision: https://secure.phabricator.com/D21368