changes qeswaphelper to shate a single, long lived transport instance
instead of opening new transports to do swaps and fetch offers.
This allows to continuosly fetch offers, so events which get returned
later by slow relays don't get missed and the fee values stay updated.
Also fixes a race causing the list to miss some swapservers, as the
current implementation fetches only until
`swap_manager.is_initialized()` is set, which will get set as soon as an
event of the configured swapserver is received. So if the event of the
configured swapserver is received as first, all server events coming in
after it would get ignored.
3a39abc415