exchangers: Improve composability of multi-threaded tx path.

src/xt_exchanger.c

src/xt_exchanger_internal.h

src/xt_exchanger_irecv_isend.c

src/xt_exchanger_irecv_isend_ddt_packed.c

src/xt_exchanger_irecv_isend_packed.c

src/xt_exchanger_irecv_send.c

src/xt_exchanger_mix_isend_irecv.c

src/xt_exchanger_simple_base.c

src/xt_exchanger_simple_base.h