Improve the shutdown functionality, such that a call to ble.shutdown() from
the user application clears the API and nRF5x state and NOT only the
SoftDevice. To achieve this the following changes are introduced:
* Add a protected member cleanup() to nRF5xGap, nRF5xGattClient,
nRF5xGattServer, nRF5xSecurityManager and nRF5xServiceDiscovery.
* Modify the shutdown() implementation in nRF5xn such that it also calls the
static member shutdown() exposed by the BLE API in Gap.h, SecurityManager.h,
GattClient.h and GattServer.h.
* Modify nRF5xGattClient, nRF5xGattServer and nRF5xSecurityManager
classes so that they dynamically create their respective objects only if
needed. Previously the GattClient, GattServer and SecurityManager objects were
declared as static, which means that they were always present even though they
were not always needed. This increases memory consumption unnecessarily.
Furthermore, pointers to the object instances are stored in static members of
the classes as specified by the BLE API base classes. This ensures that
calls to shutdown do not require calls to getInstance() functions that would
otherwise result in undesired memory allocations.
* nRF5xGap object is always needed, so this remains allocated statically. But
the reference in Gap is pointed to this object.
The shutdown procedure is as follows:
1. The user calls ble.shutdown() which executes the code in nRF5xn::shutdown()
1. The SoftDevice is shutdown
1. The static members of Gap.h, SecurityManager.h, GattClient.h and
GattServer.h are called to clean up their own state.
If at any point an error occur during the last step, BLE_ERROR_INVALID_STATE is
returned.
