S2E context is not reset between reconnection attempts

[adriansmares]

References #180

While handling router_config messages from the LNS, certain s2ctx_t fields are not reset, and in turn are kept even if the LNS returns a different router_config.

The fields which seem to be affected by this, but there may be more:

• canTx
• txpow2
• ccaEnabled

This is very visible if the LNS first returns a CCA enabling region (AS923_1 for example), then returns a region without CCA (like an invalid AS923) - station continues to enable CCA even though CCA should not be enabled for that region.