Manage distributed_context using shared pointers (#555)

* Replace distributed_contest with shared_ptr<distributed_context> in execution_context and pass around the shared pointer instead of a raw pointer.
* Fix construction of mpi_context
* Remove num_threads() from arb and arb::threading. Modify mpi_context so it also returns a shared_ptr. proc_allocation is initialized from execution context to determine available resources.
* Rename threading backend files. Delete useless files.
* Pass execution_context by const reference or value.
* Remove code duplication in thread_system constructors.