Implement useful debug logging

[cer]

First, there needs to be a good way to enable DEBUG logging, e.g. Environment variable, such as EVENTUATE_NODE_JS_CLIENT_LOGGING_LEVEL?

Second, all main API functions (AggregateRepository, Eventuate Client) need to debug log on entry (parameters) and on exit (return value)

Third, EventuateSubscriptionManager (or one of the classes that it uses) needs to debug when:

• It calls an event handler
• When the event handler returns succesfully
• When the event handler fails