raft: introduce election_mode configuration option

The new option can be one of 3 values: 'off', 'candidate',
'voter'. It replaces 2 old options: election_is_enabled and
election_is_candidate. These flags looked strange, that it was
possible to set candidate true, but disable election at the same
time. Also it would not look good if we would ever decide to
introduce another mode like a data-less sentinel node, for
example. Just for voting.

Anyway, the single option approach looks easier to configure and
to extend.

- 'off' means the election is disabled on the node. It is the same
  as election_is_enabled = false in the old config;

- 'voter' means the node can vote and is never writable. The same
  as election_is_enabled = true + election_is_candidate = false in
  the old config;

- 'candidate' means the node is a full-featured cluster member,
  which eventually may become a leader. The same as
  election_is_enabled = true + election_is_candidate = true in the
  old config.

Part of #1146