use stack allocated eventuals and mutexes where possible #34
Description
from @mdorier: we might want to use ABT_eventual_memory and ABT_mutex_memory instead of ABT_eventual and ABT_mutex. We have macros in the margo codebase to select one or the other based on availability. Using the _memory version removes an allocation, which can be good for the abt_io_io_state structure, which are allocated on each operation.