handler: read config before starting event loop

[jkarneges]

This is prep work for moving to the new event loop which requires the config in advance. The handler initialization process is refactored to use synchronous code for parsing CLI args and reading the config file, then the event loop is started and further initialization continues from there within the event loop.

Idiomatic Qt apps typically start the event loop immediately in main so that pretty much all initialization occurs within the event loop, but as we prepare to move away from the Qt event loop in the proxy and handler we can adopt the more precise style used by connmgr which only instantiates event loops when/where actually needed. During early initialization steps such as CLI arg parsing there's not much value in having an event loop, and so it's fine to wait until later to start one.