Rewrite the handling of swap

This MR does quite a few things:

• Port the existing swap handling from shell scripts to golang
• Make the cache device command handle both mounting and unmounting, thus removing even more shell code
• Add support for backing the swap space by a block device
• Add support for resuming from hibernation (hibernating is not yet supported, and will be implemented as part of #54)

TODO:

• Fix #44
• Add support for swappiness control