DRTVWR-476: Add LLEventLogProxy, LLEventLogProxyFor<T>.

LLEventLogProxy can be introduced to serve as a logging proxy for an existing
LLEventPump subclass instance. Access through the LLEventLogProxy will be
logged; access directly to the underlying LLEventPump will not.

LLEventLogProxyFor<LLEventPumpSubclass> functions as a drop-in replacement for
the original LLEventPumpSubclass instance. It internally instantiates
LLEventPumpSubclass and serves as a proxy for that instance.

Add unit tests for LLEventMailDrop and LLEventLogProxyFor<LLEventMailDrop>,
both "plain" (events only) and via lleventcoro.h synchronization.