Catch internal salt responses in mtproto

[er-azh]

Description:

I tried to fix salt_request_msg_id to ensure that salt responses are caught internally and don't reach the Sender.

My changes:

• salt_request_msg_id is now an Option<MsgId> instead of Option<i64> to match other types.
• It is assigned the message ID of the new salt request when requesting additional salts.
• A redundant check is removed.
• It gets reset to None upon a reset.

[Lonami]

Thanks!