Replace db transaction used when saving flattrs with a redux saga

[erikvold]

At the moment we use a read+write db transaction to prevent us from trying to save/write more than one flattr at a time, the down side to this is that all queries to the flattr db made while a save is in progress are delayed while they wait for the transaction to complete.

It would be more efficient to replace the transaction with a redux-saga to simply buffer the save actions, this way if there is some other process that needs to write to this db in the future then we can control the access to the db with redux-saga.