Change parent strategy during transaction

[Olsen, Brian]

Hi dev@trafficserver.apache.org<mailto:dev@trafficserver.apache.org>,

I have a PR up which allows the current parent strategy to be 
set/changed/cleared in a transaction when client request headers are being 
evaluated: https://github.com/apache/trafficserver/pull/12512

The new APIs proposed are (still done in a ‘c’ friendly style)

TSHttpTxnNextHopStrategyGet
TSHttpTxnNextHopStrategySet
TSHttpNextHopStrategyNameGet
TSHttpTxnNextHopNamedStrategyGet

The PR documents the use of these functions.

Please review and comment.

Thanks!
Brian Olsen