Make it possible to launch and track individual Janitor tasks

For now we keep the 'akonadictl fsck' and 'akonadictl vacuum' commands to execute certain checks in a batch, but we also introduce two new 'akonadictl check' and 'akonadictl migrate' commands. Just used like this they list all available tasks. 'akonadictl check:task' launches a particular maintenance task.

This opens doors for future user-controlled optional migrations that are too expensive to run automatically in the background and instead can be prompted through a notification and started, paused and stopped by user from some GUI as well as providing progress reporting.