[RPC-Spec-V2] Cancel ongoing operations

[lexnv]

Cancel any ongoing operation identified by the unique operation ID:

• chainHead_stopBody, chainHead_stopCall and chainHead_stopStorage are removed
• the chainHead_stopOperation will terminate any operation that is pending a result
• it is possible that an ongoing operation has already produced the result before the user calls this method

// @paritytech/subxt-team

[tomaka]

For what it's worth I don't think you need to implement this properly in a full node, given that you send back the results immediately. The only exception is when a chainHead_storage is in a waiting-for-continue state.