Save last message variant and time received

[ctm]

Have both the Lobby and each Table record the variant of the message they've received along with a timestamp.

Then put that info into the BrowserError, so we can get some idea of what was last processed when an error comes in. FWIW, this won't necessarily help us with dropped closures (#1523), because it may be that the closure was passed to the JavaScript subsystem in a way that only invokes the closure after a delay. However, being able to compare the client time stamp of when the socket receives a message and when the message is logged as having been sent allows us to get some idea of whether the client clock can be trusted and if not, the approximate offset to compensate by.

[ctm]

FWIW, although most messages are tiny, there are a few that are fairly large. The problem with storing only the variant is that the same variant may appear fairly frequently. We could make an enum that is either the actual message itself or the first N bytes of the message's Debug representation. That would prevent us from having to send an entire large message which should be sufficient, since the large ones aren't repeated AFAIK.

[ctm]

After sleeping on it, just sending up to N characters of the DEBUG representation, regardless of the size is less complex and easily good enough.