Fix concurrent access to signer queue

[tomusdrw]

Currently it's possible that two parallel RPC requests to with password to sign a signer queue entry will cause two transactions to be sent to the network.

The issue exists since we are running a multi-threaded RPC, and the signer queue doesn't have explicit lock while accessing an item.

Code: https://github.com/paritytech/parity/blob/7d7d4822a5087b24b49f2a1ddb111f2daa51a017/rpc/src/v1/impls/signer.rs#L95

So we peek some ID, do signing and later removed it from the queue. If nonce is not provided explicitly both threads will get a unique one and cause two same transactions to be sent.

Related #8646