Cherry-picking from Martins obj/objcore/exp patch:

move n_object accounting responsibility to stevedores.

Fix n_object accounting detail in persistent.
Make EXP_Ttl() take a pointer to struct exp rather than struct object
Move busyobj->exp to bo->fetch_objcore->exp
Divorce EXP_Inject() and EXP_Insert() -- they're quite different.
Eliminate obj->exp and use the one we have on objcore instead.

Essentially identical to patch by Martin