Correct mesh network connection status callback functionality.

[KariHaapalehto]

Now the status callback is only called if the status changed.

Description

With the 6lowpan network, lost of border router did cause a loop of NSAPI_STATUS_CONNECTING-NSAPI_STATUS_DISCONNECTED network status indication pairs.
Based on https://confluence.arm.com/display/IoTBU/NetworkInterface+status+callbacks
lost of network should only cause one NSAPI_STATUS_CONNECTING status indication callback.
With these changes only one status indication callback is called.
Tested that both 6lowpan and Thread network status callback's are now working as the should.

Pull request type

[X] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

[KariHaapalehto]

@artokin even Seppo already approved this, could you also check this

[cmonr]

Fyi:

[Build K66F ARM] [Warning] Nanostack.h@278,0: #1300-D: call_in inherits implicit virtual
[Build K66F ARM] [Error] MeshInterfaceNanostack.h@58,0: #321: data member initializer is not allowed
[Build K66F ARM] [ERROR] "./mbed-os/features/nanostack/nanostack-interface/Nanostack.h", line 278: Warning: #1300-D: call_in inherits implicit virtual
[Build K66F ARM] "./mbed-os/features/nanostack/mbed-mesh-api/mbed-mesh-api/MeshInterfaceNanostack.h", line 58: Error: #321: data member initializer is not allowed
[Build K66F ARM] ./mbed-os/features/nanostack/mbed-mesh-api/source/LoWPANNDInterface.cpp: 1 warning, 1 error
[Build K66F ARM]
[Build K66F ARM] [mbed] ERROR: "/usr/bin/python" returned error.
[Build K66F ARM] Code: 1
[Build K66F ARM] Path: "/builds/ws/mbed-os-ci_cloud-client-test/mbed-cloud-client-example"
[Build K66F ARM] Command: "/usr/bin/python -u /builds/ws/mbed-os-ci_cloud-client-test/mbed-cloud-client-example/mbed-os/tools/make.py -t ARM -m K66F --source . --build ./BUILD/K66F/ARM"
[Build K66F ARM] Tip: You could retry the last command with "-v" flag for verbose output
[Build K66F ARM] ---
[Build K66F ARM] [mbed] Auto-installing missing Python modules...

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3383
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8439/

Triggering tests

/morph test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 3016
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8439/

[mbed-ci]

Test : SUCCESS

Build number : 3186
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8439/3186