Download transaction should not block other transactions

[sidt4]

Download transaction (part of update-packages) blocks PK clients for longer periods of time, which is not a good thing.

E.g. When the user clicks Download in updates page of GNOME Software, no other PK dependent transaction works in GNOME Software until the entire update packages download is completed,

There have been discussions in the past on making PK multi-threaded, but due to PK backend limitations, this is not possible currently. Rather than pursuing multi-threading, we can just special case update-packages (download) transaction alone, and let other transactions run when package downloads are in progress.

Here are the top 30 worst-case execution times of the following 5 transactions on my system over the past couple of months.

• install-packages
• refresh-cache
• remove-packages
• search-file
• update-packages(download)

Without update-packages (download) transaction:

With update-packages (download) transaction: