TSHttpTxnClientAddrGet and TSHttpTxnIncomingAddrGet may return NULL

With the clean up rearranging to ensure SSN close occurs after TXN close (TS-4507), the API calls TSHttpTxnClientAddrGet and TSHttpTxnIncomingAddrGet may return NULL. This can occur in the case where the client connection has terminated, but the HttpSM has not yet shutdown. We now null out the reference in HttpSM to the client_vc. These calls fetch the addresses from the client_vc, so it HttpSM reference to it has been removed, these API's will return NULL.

Locally, we copy these addresses into the ProxyClientSession before the client_vc is disconnected. We had push back from deployed plugins for a short term fix.

Not clear what if anything we want to do in open source. But wanted people to be aware that this is an issue.