gmx_tx / gmx_rx replaced by shmem_sendrecv

The gmx_tx and gmx_rx has been replaced by gmx_tx_XXX_off
which calls the shmem_sendrecv_real operation.
The original tx/rx routines were not useful for the SHMEM implementation.
grid->cell_index is now allocated on SHMEM,  so that it can be used in the shmem
comm routines called from the nsgrid module (mv grid).

Tested with 4p and 8p.