papi: introduce read_blocking

papi: introduce read_blocking

Previously, self.transport.q got the messages
(at least for socket transport), stored in the encoded (packed) form.
In order to avoid accessing internals for async reads,
a new method is introduced, to perform
blocking reads of decoded (unpacked) messages.

The method is also used in _call_vpp(),
so sync and async reads are kept compatible.

Type: feature

Change-Id: Id49792dfa57c00b1a14a198031c5398d09a9ba20
Signed-off-by: Vratko Polak <[email protected]>
(cherry picked from commit 0938547eaaf5440de28877fa4f873c21bcb8cfbf)