Issue title: Feature request: multithreading building AUR packages
 Issue body: I often get a lot of AUR updates at the same time. It would be nice if these packages could be built simultaneously.
I understand that it should be impossible to install them all the time, but building and compressing them should be possible.
 Comments: 
 Comment 0: Even if trizen had all information on AUR packages beforehand (which it doesn't, see #89) this would be difficult to implement with no reasonable gain - you can already set `-j4` or similar in `makepkg.conf`. See #127.
 Comment 1: It would give a reasonable gain when having several big updates which on their own are single threaded (thinking about all the Jetbrains packages in the AUR). The download and package steps take a huge amount of time (as they have to compress ~500MB per package). These would be faster when done in parallel.
 Comment 2: You can enable multi threaded compression using the compression options in makepkg.conf.
 Comment 3: ...or you just disable compression altogether since it's quite irrelevant when not hosting packages.
 Comment 4: ```
morganamilo@Octavia ~ % df / /boot -h  
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda2       193G  178G  6.8G  97% /
/dev/sda1      1022M   93M  930M  10% /boot
```

Not all of us have huge amounts of storage ;)

But yeah you can configure the compression however you like.
 Comment 5: Storage ain't no problem here. And when it does `rm /var/cache/pacman/pkg/*` solves that problem. :)
That seems to lower the need for this feature. Will close this for now.
{'source': 'GithubIssues', 'repo_name_with_owner': 'trizen/trizen', 'issue_id': 330147987, 'repo_stars': 760.0}