[cli_wallet] race condition while saving a wallet could potentially lead to data loss

use case scenario alice love to play with Hive via cli_wallet but also loves to have a lot of windows/consoles open

starts cli_wallet instance[1]
gets distracted curating content through web interface
starts cli_wallet instance[2]
cli_wallet instance[2]: set_password xxx, unlock xxx, import_key key1, import_key key2, closes wallet, goes for lunch
gets back to pc looks at instance[1], closes wallet

so at step 4 wallet will save with two imported keys but at step 5 it will save empty wallet without keys

solution would be to use .lock file in order to make sure that the file won't be overwritten?