1. Fixing astyle and docs
2. Extra mutex lock was removed
3. Bytes are updated when send/recv > 0 and not in case of -ve error
4. Review comments
5. Guard statistics implementation in test with MBED_NW_STATS_ENABLED
From lowercase to uppercase and drops 'test_'-prefix. Test case
descriptions do also match to TC names now.
Additionally all the TCs are not behing MBED_EXTENDED_TESTS
anymore.