This is a squashed commit containing multiple changes

completion_modes:
pika supports different completion modes that may be used
as an alternative to the dlaf:: transformMPI mechanism
that uses yield_while to wait on an MPI request.

The completion modes may be set via the environment variable
PIKA_MPI_COMPLETION_MODE=<numeric value>
which by default will select the one chosen by pika/dlaf
developers known to give good results across a broad range of
use cases.

polling:
The pika polling loop may test for one or multiple request
completions on each iteration through the scheduling loop
the environment var
PIKA_MPI_POLLING_SIZE=<numeric value> (default 8)
may be used to vary the polling size (typically the default value
can be used without any need to play with this value)

mpi pool: pika will create the mpi pool if the completion mode
has the pool flag set, the user needs only to call the pool create
function during the pika::init setup phase.
Cleanup of the pool on shutdown will also be handled automatically

The user should use pika::mpi::pool_name instead of raw "mpi",
mpi pool management has been deferred tom pika::mpi

Change: the transform mpi code does not need to return an MPI_SUCCESS
value, the return value from mpi_transform has been removed to simplify
code and an error is set using senders set_error if any mpi call fails.
Should mpi_transform calls thnat return other value be required, this code
can be reinstated.

A series of changes to pika::mpi have changed both the API and the
internal pool creation mechanism to simplify end user access to the
transform_mpi features and setup of mpi polling itself.

latest pika:: transform_mpi is enabled/disabled via the command line
and does not require explicit enabling via init_params