Begin fetching the current recommended gas price #792

If we keep using the web3 crate even after #GH-676 has been played, or somehow otherwise if it's a different library, we'll want to call the web3 JSON RPC eth_gasPrice.
In the current library there is a method under the Eth module called like so, gas_price().

This should be called in the process of constructing the BlockchainAgent, which is a place where we are also fetching this value now but only by reading a value in the DB. Such a value is probably outdated by that time and almost never changing, though. We want a dynamical value corresponding tightly to the blockchain network's demand at the moment so that the transaction can have good chances for getting picked by the miners.

There should be another parameter, gas price margin. That one, for change, should be user-oriented, and arbitrarily settable. Card #793 talks about that implementation. This margin will be denominated in percents of the recommended price as an extra to that price.