fix: concurrent calls to txProposalCreate might lead to the same tx_outputs being locked for multiple tx_proposals

[andreabadesso]

This issue originated from @luislhl comment:

What happens in case 2 txProposals using the same inputs are created concurrently, and both pass this check? This is a possible scenario, right?

In which future step do we detect the duplicity?

I'm asking this more out of curiosity than anything

Originally posted by @luislhl in HathorNetwork/hathor-wallet-service#82 (comment)