Improve shutdown to clear BLE API and not just SD

[andresag01]

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()
2. The SoftDevice is shutdown
3. 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.

NOTE: This enhancement also requires changes to the ble module @rgrover @pan- @LiyouZhou

[andresag01]

@rgrover @pan- I have made changes according to comments in this pull request.

[andresag01]

@rgrover @pan-: Made changes according to comments.

[andresag01]

@rgrover made changes to make _gapInstance non-static

[rgrover]

@andresag01 I'm happy with this. Will wait for @pan-

[pan-]

It's alright, I'm looking for the next PR including onShutdown

[rgrover]

ble-nrf51822-2.2.7