[MPI] Const-incorrectness in boost::mpi::status

[daniel_kruegler]

There are two member functions in class boost::mpi::status which lack const qualifications although they are immutable:

(a) bool cancelled(): The documentation says: "Determine whether the communication associated with this object has been successfully cancelled." The implementation delegates to MPI_Test_cancelled, which according to the internet documentation does use it's MPI_Status* argument is IN parameter.

(b) template<typename T> optional<int> count(): Here also the docs don't imply a mutable operation. The implementation delegates (under some circumstances) to MPI_Get_count, which also does not modify MPI_Status from a user perspective. The same function *caches* the result in the data member m_count, but this is the classic example where this member should be declared mutable therefore.

[Douglas Gregor]

(In [39052]) Constify status::count and status::cancelled. Fixes #1101