Enable bulk requests for payment notes

[louilinn]

Is your feature request related to a problem? Please describe.
At the moment it is only possible to request one payment note at a time. For the wallet statement export we need many payment notes during the export period. Requestibg the api 50+ or 100+ times slows down the export time.

Describe the solution you'd like
Enable requestibg payment notes based on an array of txhashes. Respond with txhash strings mapped to payment notes. For payment notes not in the db, dont include the hash in the response. Do not fail if tx hashes are missing in db.

Describe alternatives you've considered
Continue calling the api many times for each export.

Additional context
Issue number #623 in myxogastria

[louilinn]

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