res_pjsip_caller_id: Use static pj_str_t for fromto header names.

PJSIP assumes that these header names are not allocated, does not clone
the name strings when reusing headers.

Block unload of res_pjsip_caller_id until shutdown to ensure static
memory stays valid.  It was previously unsafe to unload while any
sessions are active.

Change-Id: I190854dea943d6e441cf03733f8a0da661aea27f