raft: use vclock_map_t for voting tracking

We've VCLOCK_MASK constant which limits the number of
regular nodes in a replication cluster. This limit is
bound to the vclock_map_t bitset type. Thus when we're
tacking voting process in Raft node we better use this
type for vote_mask member (otherwise it is a room for
error if we ever change VCLOCK_MASK and extend the width
of a bitset).

Suggested-by: Vladislav Shpilevoy <v.shpilevoy@tarantool.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>