Move allocation-related types to mesos::master namespace

Allocator, Sorter and Comaprator types live in master::allocator namespace. This is not consistent with the rest of the codebase: Isolator, Fetcher, Containerizer all live in slave namespace. Namespace allocator should be killed for consistency.

Since sorters are poorly named, they should be renamed (or namespaced) prior to this change in order not to pollute master namespace.