applier: ignore ER_UNKNOWN_REQUEST_TYPE for IPROTO_VOTE

IPROTO_VOTE command (successor of IPROTO_REQUEST_VOTE) was introduced in
Tarantool 1.10.1. It is sent by an applier to its master only if the
master is running Tarantool 1.10.1 or newer. However, the master may be
running a Tarantool version 1.10.1 that isn't aware of IPROTO_VOTE, in
which case the applier will fail to connect with ER_UNKNOWN_REQUEST_TYPE
error.

Let's fix this issue by ignoring ER_UNKNOWN_REQUEST_TYPE received in
reply to IPROTO_VOTE command.