blecent_read_write_subscribe() function

Performs three GATT operations against the specified peer: 1. Reads the ANS Supported New Alert Category characteristic. 2. After read is completed, writes the ANS Alert Notification Control Point characteristic. 3. After write is completed, subscribes to notifications for the ANS Unread Alert Status characteristic. If the peer does not support a required service, characteristic, or descriptor, then the peer lied when it claimed support for the alert notification service! When this happens, or if a GATT procedure fails, this function immediately terminates the connection.