acquire: Use priority queues and a 3 stage pipeline design

Employ a priority queue instead of a normal queue to hold
the items; and only add items to the running pipeline if
their priority is the same or higher than the priority
of items in the queue.

The priorities are designed for a 3 stage pipeline system:

In stage 1, all Release files and .diff/Index files are fetched. This
allows us to determine what files remain to be fetched, and thus
ensures a usable progress reporting.

In stage 2, all Pdiff patches are fetched, so we can apply them
in parallel with fetching other files in stage 3.

In stage 3, all other files are fetched (complete index files
such as Contents, Packages).

Performance improvements, mainly from fetching the pdiff patches
before complete files, so they can be applied in parallel:

For the 01 Sep 2016 03:35:23 UTC -> 02 Sep 2016 09:25:37 update
of Debian unstable and testing with Contents and appstream for
amd64 and i386, update time reduced from 37 seconds to 24-28
seconds.

Previously, apt would first download new DEP11 icon tarballs
and metadata files, causing the CPU to be idle. By fetching
the diffs in stage 2, we can now patch our contents and Packages
files while we are downloading the DEP11 stuff.

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