Initial version of the setCCCD API for GattClient

[andresag01]

This is an initial version of an API that automates the process of setting the
Client Characteristic Configuration Descriptor (CCCD) to enable/disable
notifications. The new API is DiscoveredCharacteristic::setCCCD().

The new API internally triggers a characteristic descriptor discovery procedure
to find the handle of the CCCD. If this succeeds, it attempts to execute a write
procedure. On termination, a user configured callback is executed to report the
final outcome of the process.

NOTE: This pull request is NOT READY FOR MERGE, it is an early version of the new API to start discussion around this issue. In my opinion, there are still a few problems that need to be tackled:

1. Shutdown should gracefully terminate the setCCCD procedure.
2. CCCDSetter::launch (and OneShotWriteCallback::launch, OneShotReadCallback::launch as well) do not return ble_error_t, so its impossible to check whether they actually succeeded.

@pan- Please comment.

[andresag01]

The build will fail because there is a problem in the API when registering onShutdown callbacks. The signature of the function should be void onShutdown(T *objPtr, void (T::*memberPtr)(const GattClient *)) rather than void onShutdown(T *objPtr, void (T::*memberPtr)(void))

I have made a PR to solve this issue: #174

[andresag01]

It seems that the ble-nrf51822 implementation of the API does shutdown the Service Discovery but NOT the Characteristic Descriptor Discoverer. I have raised an issue about this, see ARMmbed/ble-nrf51822#110. In this case, the CCCDSetter objects will never receive the expected callbacks and the memory resources allocated will never be cleared.