Consolidate the duplicated WebSocket frame-parsing logic from Read
and Write into a shared processFrames loop, fixing several bugs in
the process:

- Mixed control and data frames in a single Read/Write call buffer
  were not handled: a control frame would cause merged data frames
  to be skipped.
- Multiple data frames into one Write call weren't being correctly
  parsed: only the first frame was processed, ignoring the rest in
  the buffer.
- msg.isFinalized was being set before confirming the fragment was
  complete, so an incomplete msg fragment, could've been sometimes
  marked as finalized.
- Continuation frames without any payload were being treated as if
  they didn't have stream ID, even thought the id is already known
  from the initial fragment.

Fixes tailscale/corp#39583

Signed-off-by: Fernando Serboncini <fserb@tailscale.com>