Add to API to get a strategy during TSRemapNewInstance

[traeak]

This gives the API access to the RemapConfig url_mapping during TSRemapNewInstance calls. This contains both the assigned stragies pointer and a pointer to the loaded strategy factory.

This modifies the strategies ts API from the previous PR.

API is:

valid during TSRemapNewInstance:

void const *TSRemapNextHopStrategyFind(const char *name);
void const *TSRemapNextHopStrategyGet();
void TSRemapNextHopStrategySet(void const *strategy);

valid during remap (the Find call is changed):

void const *TSHttpTxnNextHopStrategyFind(TSHttpTxn txnp, const char *name);
void const *TSHttpTxnNextHopStrategyGet(TSHttpTxn txnp);
void TSHttpTxnNextHopStrategySet(TSHttpTxn txnp, void const *strategy);

with utility function to get the strategy name:

char const *TSNextHopStrategyNameGet(void const *strategy);

The header_rewrite and regex_remap plugins are modified to look up named strategies during TSRemapNewInstance instead of performing strategy factory lookups during each transaction remap hook.