otr: make verification service responsible for canceling verification

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
otr: make verification service responsible for starting verification

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>
otr: added OtrSessionService

Signed-off-by: Rafał Malinowski <rafal.przemyslaw.malinowski@gmail.com>