Without filtering the user poll on the full buffer which may not reflect
the real status from its point of view (with filters).

The following use case is buggy:

- buffer is not empty
- user does poll()
- poll sais that the user can read
- user does read()
- read return nothing because the buffer does not contain valid messages

This is critical for sync messages.

Signed-off-by: Federico Vaga <federico.vaga@cern.ch>