MINOR: Allow inlining of `stream_generate_unique_id()`

MINOR: Allow inlining of `stream_generate_unique_id()`

With the introduction of the `generate_unique_id()` helper, the actual
complicated logic is sitting in a different file. Allow inlining of
`stream_generate_unique_id()`, so that callers can benefit from an abstraction
without hiding away the access of `strm->unique_id` behind a function call.