[COIN 584] [XTZ] Improve Fees and GasLimit using a RPC node

[gagbo]

Fix

Compute the "mean fees" of last block by actually dividing the total fees by the number of non trivial ops

Improvements to gasLimit and fee estimations

• HttpClient : Send error payloads as well as the http code from Http exceptions (because it make debugging easier down the road)
• TezosLikeTransactionApi : Add a JSON serializer specific for the run_operation endpoint on a RPC node. The binary serialization did not work well with the endpoint and the error messages are harder to act on
• TezosLikeExplorers : add interface (implemented only on TZSTATS type of explorer) to fetch the current gasPrice
• TezosLikeExplorers : add interface (implemented only on TZSTATS type of explorer for now but can be improved) to fetch the gasLimit for a transaction through Node simulation. The code only works for tx that use Reveal and/or Transaction type operations for now
• TezosLikeAccount : give an interface to fetch the current gasPrice on the network (will throw on non tzstats explorers), and allow the transaction builder to dynamically compute the fees for a transaction.

To make the automatic estimation this is currently an admittedly bad hack to keep the current request API backwards compatible:

• the signal is to put the GasLimit to "0" in the request
• the fee price you want to pay should be in the Fees of the request, in picoTez (10^-12 XTZ) per gasUnit ; if you also leave "0" there, then the transaction builder will fetch the gasPrice from the next block (which can be 0 for an empty block)

Then on transaction building, Core detects the 2 info to actually make a transaction with the correct gasLimit and the associated fees according to your fee price. Note that there are no guarantees that the fees amount won't be too high for the sender's balance, callers should check this before signing/broadcasting.

[ghost]

Why do we need to put a fake signature here / what happens if we don't? (Tezos noob question)

[gagbo]

the node refuses to make the operation simulation with an error

[ghost]

Why not just return the result of api::BigInt::fromDecimalString here?

[gagbo]

because when I use BigInt->toInt64 later I don't know if I get 500 or 50 for a value like "0.50"

[ghost]

(Only the comment on TezosLikeTransactionApi.cpp needs double checking IMO, the rest is probably unimportant).

[ghost]

Seems all good, for a non-Tezos-expert though.
Thanks for the answers to my questions.

[gagbo]

HODL it blocks reveal for now

[gagbo]

HODL it blocks reveal for now

All issues are fixed and accounted for, but for the time being it's a pile of hacks in TezosLikeExtendedPublicKey::toBase58 and this needs to be fixed properly and defined in unit tests

[gagbo]

TODO: investigate on which branches this code is live