Allow Lace to work with foreign threads

[alaarman]

This allows the use of Lace in e.g. LTSmin:

lace_init_static(HREpeers(), dqsize);
hre_init_and_spawn_workers (argc, argv);
/*************************************************************************/
/* from this location on, multiple workers execute the code in parallel  */
/*************************************************************************/

lace_init_worker (HREme(HREglobal())); // catches slaves

CALL(reach_do, ctx->initial->ref);