Proposed new ts api calls

[Walt Karas]

const char *TSIRemapFromUrlStringGet(const TSHttpTxn txnp);

const char *TSIRemapToUrlStringGet(const TSHttpTxn txnp);

These would return, from the transaction state object,
url_map.getFromURL() and url_map.getToURL() respectively, in strong
format.  A null pointer would be returned if the values were empty.