Fix possible unsynced inputs/outputs during inserts

[tdroxler]

As described here

It's an edge case that could occurs if someone request data between the insert of the outputs and the update of the inputs.

The issues comes from here where we insert blocks info and update inputs in two phases, because insertBlocks is a future, because if a parent is missing, we need to download it before inserting the others ones, so we need a Future and can't use only DBAction to make them transitionally.

One idea to fix that: We should maybe update the input directly when inserting blocks here, but we need to make sure we can't update an input of an output that isn't inserted yet.