Changed UprRequestStream() API to include opaque.

16 LSBs are used by upr-client to differentiate between vbuckets.
16 MSBs are meant for application and available via UprEvent.

patchset-2:
- updated client/upr_example/upr_example.go

patchset-3:
- comments

Change-Id: I5a3cc485a9a038d85f04b41ec4aba350f635187f