OPENTHREAD_CONFIG_NCP_SPINEL_RESPONSE_QUEUE_SIZE macro

@def OPENTHREAD_CONFIG_NCP_SPINEL_RESPONSE_QUEUE_SIZE Size of NCP Spinel command response queue. NCP guarantees that it can respond up to `OPENTHREAD_CONFIG_NCP_SPINEL_RESPONSE_QUEUE_SIZE` spinel commands at the same time. The spinel protocol defines a Transaction ID (TID) as part of spinel command frame (the TID can be a value 0-15 where TID 0 is used for frames which require no response). Spinel protocol can support at most support 15 simultaneous commands. The host driver implementation may further limit the number of simultaneous Spinel command frames (e.g., wpantund limits this to two). This configuration option can be used to reduce the response queue size.