The Group objects were not reset correctly when attempting to replay
them, leading to confusing error log messages and invalid datapath
state. We fix the implementation of Reset() for groups and we ensure
that the method is called during replay.

We also update the TestOVSFlowReplay e2e test to make sure it is more
comprehensive: instead of just checking Pod-to-Pod connectivity after a
replay, we ensure that the number of OVS flows / groups is the same
before and after a restart / replay. We confirmed that the updated test
fails when the patch is not applied.

Fixes antrea-io#2127