mpi: adding some code to begin optimistic mode

Adding code for the optimistic mode. We split the mpi header in
mpi-synchronous and mpi-timewarp.

We remove the useless dbg header.
main: removed
context: fix bug in priority assignation

In function set_log_priority, the min and max are swapped.
vle: add newline in vle_debug message

The vle_debug function does not add a newline by default (old behaviour
of the vle::dbg).
generic: fixing integer conversion warning

Port are size_t instead of int.
generic: adding more vle_dbg trace

We add three vle_dbg into generic's post function.