download: Add support for http long polling

Earlier the downloader package only supported the http
short polling technique where the client sends periodic
requests to the server to fetch bundles. A drawback of this
method is that a low polling frequency could add unnecessary
burden on the server and network.

This commit adds support for http long polling which helps
to minimize server/network resource usage and also reduces
the delay in delivery of updates to the client.

Signed-off-by: Ashutosh Narkar <anarkar4387@gmail.com>