Add mutexes to protect access to PkClient and PkTask

This fixes the GUI hanging in an unresponsive state. The libpackagekit-glib
library isn't as thread safe as we'd like, and although we can fix PK to be
safer we can't depend on that very new version just yet.
1 job for <span class="ref-name">wip/hughsie/packagekit-thread-safety</span> in 4 minutes and 31 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #213083
build-gnome-software

00:04:31