activity: record activity on echo reply
activity: remove unused optional argument

This was not part of the public interface.
activity: rework handshake/activity distinction

Treat the handshake state as something separate and orthogonal to
tracking activity. Now it's possible to detect an idle connection that
has not completed a handshake and kill it if necessary.
activity: record activity for all messages
activity: Make echo the first stage of the pipeline

According to the OpenFlow specifications, controllers should implement
the echo response at the lowest-level possible.
Merge pull request #155 from frenetic-lang/activity

connection activity fixes