Double spend check (#606)

* Fix: added a check for doublespends

* Fix: added a check for double spends

* Fix: fixed iport cycle

* Feat: replaced error with error variable

* Feat: store tangle in factory

* Update dapps/faucet/packages/faucet.go

Co-authored-by: Wolfgang Welz <welzwo@gmail.com>

* makes send tx web API handler synced

Co-authored-by: Hans Moog <hm@mkjc.net>
Co-authored-by: Wolfgang Welz <welzwo@gmail.com>
Co-authored-by: Luca Moser <moser.luca@gmail.com>