Review DroppedPackets statistic usage/definition

[arthurpi]

The tcpip.Stats.DroppedPackets statistic is used inconsistently. It is sometimes incremented when a malformed packet is received (we have a stat for that already), sometimes when an unexpected packet is received (for example, an ack when none were expected), sometimes when a recv queue is full, sometimes when a send queue is full, etc.

We should either:

• move it in the correct stats sub types (eg tcpip.Stats.TCP, UDP, etc.)
• delete it
• make it more specific