iproto: write push responses to flight recorder

Apparently, push responses were not considered when designing flight
recorder: write push responses to flight recorder immediately when a push
is initiated (i.e., synchronously).

Needed for tarantool/tarantool-ee#338

NO_CHANGELOG=<affects EE feature>
NO_DOC=<bugfix>
NO_TEST=<tested in EE PR>

(cherry picked from commit c71bfcfa)