fiber_func returns int

src/box/applier.cc

src/box/iproto.cc

src/box/memtx_engine.cc

src/box/recovery.cc

src/box/relay.cc

src/box/wal.cc

src/cbus.c

src/fiber.c

src/fiber.h

src/lua/fiber.c

src/lua/init.c

test/app/module_api.c

test/unit/coio.cc

test/unit/fiber.cc

test/unit/fiber_stress.cc

test/unit/ipc.cc

test/unit/ipc_stress.cc