Support dynamic adaptation plans that cover multiple vectoring points.

Support dynamic adaptation plans that cover multiple vectoring points.

The dynamic adaptation plan is specified using X-Next-Services ICAP
header or eCAP meta-info, as usual. A REQMOD adaptation service may
construct an adaptation plan that starts with REQMOD and ends with
RESPMOD. Multiple adaptations may be planned at each point.

The natural transaction handling order must be preserved: the plan
cannot go from RESPMOD back to REQMOD.

Adaptation::History object is used to keep future plan steps when
crossing vectoring points.