Refactor `PIKA_MPI_COMPLETION_MODE` into independent flags

[msimberg]

While for testing the current bitmask passed to PIKA_MPI_COMPLETION_MODE is convenient, the values are not very transparent to a user setting the values. It'd be nice to split up the flags into PIKA_MPI_INLINE_CONTINUATION, PIKA_MPI_DEDICATED_POOL etc. If still needed for testing we can consider keeping some way to override the mode with a bitmask as well.