box: add OS stubs for flight recorder requests

This patch introduces two stubs: one to log incoming requests and
another one to log outgoing responses. Log itself takes place in TX
thread, in function family `tx_process_*`. At the start of TX processing
we are able to use raw msgpack from input buffer to log it as it was
received. In the same way we can log raw msgpack for responses (before
sending message to IProto thread).

NO_DOC=<No user visible changes>
NO_TEST=<No functional changes>
NO_CHANGELOG=<No functional changes>