Limit parallel ops

Operations like installation/update/upgrade-download can be resource intensive in slower machines, e.g. several installs at the same time may use memory or disk to a point where the whole system becomes unusable. This has been observed on machines that have 2GB of RAM (or less) and spinning disks.

To avoid such situations, this MR limits the number of operations that can run in parallel by using a thread pool. The number of parallel operations allowed is in function of the RAM (one thread allowed per GB).

While actions are pending, the details page will show an empty progress bar and a label with the explanation of what's going on. The UX for the QUEUED_FOR_INSTALL state has also been changed in order to match the one of the pending actions.