Provide a thread if the translator wasn't passed one in the request headers

Provide a thread if the translator wasn't passed one in the request headers

The OpenSRF-over-HTTP spec says that the translator will generate a thread
in the response if one wasn't passed in the request; however, we were trying
to pass a null pointer back and that doesn't work so well.

As we're already generating a reasonably unique string for the purposes
of delim in the multipart data response, let's also use that for the
response thread when necessary.

git-svn-id: svn://svn.open-ils.org/OpenSRF/trunk@1906 9efc2488-bf62-4759-914b-345cdb29e865