add an option to purge the transfer history

Add this feature to the REST API
Add a command to the client that use it

Purge should only delete history entries by default.
Add an option to reset transfer ID increment. This option requires that the transfers table is empty.

Related: #74 (closed) (history export) to have the possibility to export what is purged

Edited Sep 08, 2021 by Maxime Robert

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