Change how RPCs eth_call and eth_estimateGas handle "Pending" (#11127)

* Change how RPCs eth_call and eth_estimateGas handle "Pending"

Before this PR we would return a rather confusing error when calling `eth_call` and `eth_estimateGas` with `"Pending"`, e.g.:

```
{"jsonrpc":"2.0","error":{"code":-32000,"message":"This request is not supported because your node is running with state pruning. Run with --pruning=archive."},"id":"e237678f6648ed12ff05a74933d06d17"}
```

In reality what is going on is that users often use `"Pending"` when they really mean `"Latest"` (e.g. MyCrypto…) and when the block in question is not actually pending. This changes our behaviour for these two RPC calls to fall back to `"Latest"` when the query with `"Pending"` fails.
Note that we already behave this way for many other RPCs:

	- eth_call (after this PR)
	- eth_estimateGas (after this PR)
	- eth_getBalance
	- eth_getCode
	- eth_getStorageAt

Closes https://github.com/paritytech/parity-ethereum/issues/10096

* Fetch jsonrpc from git

* No real need to wait for new jsonrpc

* Add tests for calling eth_call/eth_estimateGas with "Pending"

* Fix a todo, add another

* Change client.latest_state to return the best header as well so we avoid potential data races and do less work

* Impl review suggestions

* Update rpc/src/v1/impls/eth.rs

Co-Authored-By: Niklas Adolfsson <niklasadolfsson1@gmail.com>

* Review grumbles

* update docs

ethcore/src/tests/client.rs

@@ -17,6 +17,7 @@
 use std::str::{FromStr, from_utf8};
 use std::sync::Arc;
 
+use account_state::state::StateInfo;
 use ethereum_types::{U256, Address};
 use ethkey::KeyPair;
 use hash::keccak;