xrow: split iproto_reply_vote into helpers

Extract mp_sizeof_ballot_max() and mp_encode_ballot() helpers from
iproto_reply_vote(), since they will be used by builtin
"internal.ballot" event soon.

While I'm at it, fix mp_sizeof_ballot() calculation: add forgotten map
element and replace mp_sizeof_uint(UINt*_MAX) with sizes of actual
values to be encoded.

Prerequisite #5272

NO_CHANGELOG=refactoring
NO_TEST=refactoring
NO_DOC=refactoring