Simplify the move from sp->wrk to sp->obj accordingly.

Add a diagram that explains how ttl, grace & keep related to
each other.

Fix VCL's obj.ttl variable to appear to be relative to "now" rather
than obj->entered. (#956)

Also log SLT_TTL when we change obj.grace and obj.keep

Make SLT_TTL always have the same format:
	XID
	"RFC" or "VCL"
	obj.ttl
	obj.grace
	obj.keep
	obj.entered
	obj.age
In addition "RFC" has:
	obj.date
	obj.expires
	obj.max-age

Fixes	#956

Thanks to:	DocWilco