fix: parent-child ipc communication

When picodata process forks, the parent and child are connected by
two unix pipes:
- the fuse (used by the child to detect parent's death);
- the ipc channel (used by the parent to get on-exit messages from
  the child to change its state in the finite-state machine);

The problem was that the parent tried to read the message from the
pipe right after the fork (and before the child's exit). Usually
the pipe was empty at that moment and the parent collected serde's
"failed to fill whole buffer" error instead the real message provided
by the child.