Replace the gas-price configurable parameter with gas-price-margin  #793

GH-598 Third stage

Introduction:

The times when we thought that the user should specify the gas price himself is gone.
Instead, the preferred way is to fetch the oracle price from the blockchain. That value should then be increased by a margin.

Regarding the margin alone:

The payment process, the way we regard it now, should contain a parameter which will affect by how many percents
a recommended gas price, fetched from the blockchain service provider, should be increased in order to achieve maintaining a high rate of passes in our txs. That way, they will have as much gas that will work even after the short delay between the PaymentAdjuster's analysis (also possibly a conversation with the Neighborhood) and the actual payment stage.

Another concern to mitigate by this parameter lies in the batch structure of our RPC request. Given the possibility of processing multiple payments at once, we don't want to open up an opportunity for any of those to fail because that would mean all transactions after this one would fail too, if nothing else, just for the reason of their suddenly invalid nonces.

When the BlockchainAgent is being constructed this value should be read from the database.

Operatives:

The user interface must be reduced by this gas-price parameter, both on the level of the Daemon and the CLI. Internally, the corresponding entry should be removed from the config table. Also remove the gas price default value (which has been mistreated for long anyways) and the code in charge for its processing. On the contrary, the other parameter should be added at the same places. It sounds like a good opportunity for one by one replacement, modifying the existing parameter instead of deleting it and adding a new one just after.