Allow apps to control logging used in getAgent / agent-proxy

[kriswest]

Enhancement Request

As discussed at meeting #1478, applications should be able to control what logging is exposed by getAgent and the agent-proxy (where getAgent has returned a DesktopAgentProxy linked to a browser-based DA) in the app's JavaScript console.

There are a number of different types of log statement currently, that should be individually controllable:

• log-level connection messages from getAgent (default on)
• debug level connection message from getAgent (default off)
• debug-level logging from the DesktopAgentProxy (default off) - on all API calls message exchanges.
• debug-level heartbeat messages from the DesktopAgentProxy (default off)

Error and warning messages should remain always logged.