Backport upstream's impl to clear engine signer.

[afck]

Original commit messages:

RPC method parity_clearEngineSigner

Add RPC method parity_clearEngineSigner
Fixes #113

corrected the return type of clear_author

review comment responses and a rebase fix

removed a spurrious warning

moved clear_signer functionality to set_signer

Merge clear_author into MinerService::set_author.

Add trace logs to Clique::set_signer.

Clique: Don't lock signer multiple times.

[varasev]

Works fine, but there is no info message in node's logs about clearing the engine_signer.

Seems all the similar RPC JSON requests don't report anything into the logs. I'd propose to add an info log message to the change_field function:

parity-ethereum/ethcore/types/src/header.rs

Line 323 in b52ac20

fn change_field<T>(hash: &mut Option<H256>, field: &mut T, value: T) where T: PartialEq<T> {

Since openethereum#10920 is already merged, let's maybe propose that in a separate upstream's PR.