Introduce IPROTO_VOTE command

The new command is supposed to supersede IPROTO_REQUEST_VOTE, which is
difficult to extend, because it uses the global iproto key namespace.
The new command returns a map (IPROTO_BALLOT), to which we can add
various information without polluting the global namespace. Currently,
the map contains IPROTO_BALLOT_IS_RO and IPROTO_BALLOT_VCLOCK keys,
but soon it will be added info needed for replica rebootstrap feature.

Needed for #461