It allows to invoke native contracts as committee
from CLI in privnet, e.g. to set new oracle nodes.
Also don't require `out` flag in `multisig sign`
if tx is to be pushed.
Follow missed change from neo-project/neo#1816 .
`None` may be used for any signer. Currently it is used
for sender to only pay fees, or to sign tx attributes.
If port is dynamically allocated, `(*Server).Addr` will contain
0 port. This commit executes listener before exiting from `Start()`
and sets Addr to the actual address.