Lock in-use keys to prevent nonce race condition

[encody]

Currently, payloads are signed using a KeyRotatingSigner as soon as they are ready to be relayed. However, this results in a race condition wherein keys that have in-transit transactions (which will update the corresponding key's nonce upon resolution) may be used to sign multiple payloads before the one resolves, causing a nonce race condition.

One way to solve this would be to put all of the keys in a key pool. When a signature request arrives, the handler requests a lock on a key from the pool. If all of the keys in the pool are locked, then the handler waits until a lock is released. A key lock is released after a payload has been signed, transmitted to the network, finalized in a block, and the key's nonce has been appropriately updated.