evaluate metatx

[Ivshti]

https://metatx.io/

related to https://github.com/AdExNetwork/adex-dapp/issues/23

also related to the new protocol (adex-protocol-eth)

considerations:

• how does it play with our SC calls where msg.sender does not matter; in that case, we only need someone to submit a tx from any pk
• how easy it is to integrate
• how can it help us with identity/login

[Ivshti]

closing cause of https://github.com/AdExNetwork/adex-protocol/issues/10