Implemented synchronous and asynchronous sending

Implemented synchronous and asynchronous sending

The main goal of this big change is to avoid delays caused by CPU
scheduler when rescheduling from application thread to VRES TX thread.
According to our fwp-timing.c experiment, these delays can by even
several milliseconds long.

Now, whenever VRES capacity allows, send operation is invoked directly
from application thread. Only if the budget is insufficient, the
message can be queued for sending later by VRES thread, provided that
asynchronous send was requested. In case of synchronous send, the
application thread is blocked until the budget is replenished.

Besides the above change, the code was cleaned up a lot.

Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>