refactor: bec_lib, part 1

For #97 (closed)

This is the first bunch of refactoring changes. Currently, I'm just happy to have the structure of bec_lib flattened and not run into circular import issue at the same time. Besides that,

• the module BECMessage is renamed to messages, so it's not possible to confuse it with the BECMessage class located in the same module
• channel_monitor.py and init_config.py scripts are moved into bec_lib/util_scripts folder, note that bec_lib/scripts is reserved for user defined scripts!
• session_manager and singleton_threadpool are removed
• Alarms enum is moved from redis_connector to alarms_handler

These changes to the bec_lib should be backwards-compatible, for example, they shouldn't break the current version of orphyd_devices and/or bec_widgets. It's achieved via module stubs in old locations, silently importing from new locations. The solution is fine, but should be only a temporary one. I plan to prepare the appropriate modifications to all libraries that currently depend on bec_lib in parallel in order to have a smooth transition. Though, it might be the best to do it after all refactoring is in place.

The size of merge request is large, but mainly due to test_config.yaml shuffle. Also, because of these module stubs, gitlab didn't detect that modules were moved with minor modifications :\ Each individual commit, however, is rather small.