`ExtendedQueryStateMachine.cancel` cancels promises without putting backend back in idle state

[ahoppen]

In my opinion, the following code does not make sense because it cancels promises without putting the backend back into the idle state, which leaves the backend and frontend state out of sync. In practice this doesn’t matter since we can’t cancel a query before having a row stream so this code should never be called.

postgres-nio/Sources/PostgresNIO/New/Connection State Machine/ExtendedQueryStateMachine.swift

Lines 106 to 112 in c17db2f

	self.isCancelled = true
	switch queryContext.query {
	case .unnamed(_, let eventLoopPromise), .executeStatement(_, let eventLoopPromise):
	return .failQuery(eventLoopPromise, with: .queryCancelled)
	case .prepareStatement(_, _, _, let eventLoopPromise):
	return .failPreparedStatementCreation(eventLoopPromise, with: .queryCancelled)