Review information emitted in contract events

[kevsul]

Let's review the information contained in the events emitted by the contracts, especially the exit-related events as described here: #576

We've discovered that relying on transaction calldata is a bad idea https://github.com/omgnetwork/private-issues/issues/64

Also, this issue is related #573

[InoMurko]

There's quite a few issues related to Events now. Perhaps we should have a larger scope for all these

#573

#576

#129

[InoMurko]

Another one not particulary useful is:

emit ChildChainActivated(authority);