Move to lwip netconn api to correctly synchronize multiple threads

- lwip socket api did not provide signaling mechanism compatible with
  mutliple stacks
- lwip raw api did not support multiple threads

The netconn api provides the necessary signalling mechanism while still
supporting multiple threads. The netconn api also shares several design
similarities to the current socket api.

Additionally, the move to a higher-level api reduced implementation
complexity significantly and will hopefully reduce integration
difficulties

Author: geky