Support reliable, cross-host, cross-process backtraces

See https://github.com/djs55/xen-api/blob/experimental-exceptions/docs/design/backtraces.md

This needs to be simultaneously merged with:
- [xapi-project/xen-api-libs-transitional#14]
- [xapi-project/stdext#9]
- [xapi-project/xenopsd#147]
- [xapi-project/xen-api#1999]
- [xapi-project/xcp-rrdd#24]
- [jonludlam/vhdd#7]
- xen-dist-ocaml.hg: not on github (yet)

NB this removes the manual 'name_thread' functions which were leaking in a few places. Please now use Debug.with_thread_named.