CLI command to delete specific builds from the disk

[agis]

Deleting a build in mistry (e.g. for freeing up disk space) is not a straightforward task:

1. it's filesystem-dependent
2. it has to be done with care with regards to the latest link (i.e. builds pointed to by the latest link should not be deleted)

For this reason, we should have a CLI command in the server binary (mistryd) that is aware of the above and can be used to easily delete specific builds.

Something like:

$ mistryd delete-build <build-id>