More or less just to avoid orders the user wasn't aware of from
persisting until we get "open order relaying" through the ems working.
Some further fixes which required a new `reqids2txids` map which keeps
track of which `kraken` "txid" is mapped to our `reqid: int`; mainly
this was needed for cancel requests which require knowing the underlying
`txid`s (since apparently kraken doesn't keep track of the "reqid" we
pass it). Pass the ws instance into `handle_order_updates()` to enable
the cancelling orders on startup. Don't key error on unknown `reqid`
values (for eg. when receiving historical trade events on startup).
Handle cancel requests first in the ems side loop.
caecbaa231