reconnect: Rename reconnect_received() to reconnect_activity().

Receiving data is not the only reasonable way to verify that a connection
is up.  For example, on a TCP connection, receiving an acknowledgment that
the remote side has accepted data that we sent is also a reasonable means.
Therefore, this commit generalizes the naming.

Also, similarly for the Python implementation: Reconnect.received() becomes
Reconnect.activity().

Signed-off-by: Ben Pfaff <blp@nicira.com>