Proposal: Refactor the codebase to reflect internal service boundaries

[macobo]

As I've worked on the plugin-server codebase the hardest thing for me has been keeping a clear view of how the program functions for two reasons:

1. It's hard to tell if a piece of code is called within the main thread, workers or both
2. It's hard to map out the message flow due to same or similar-meaning verbs being used all over the place: For example, what's the difference between ingesting and processing events, saveEvent/ingestEvent naming getting changed, processEventBatch being both a worker task name as well as a plugin method name etc.

Our codebase is currently split up up by functionality - e.g. one folder for ingestion-related logic, another for worker setup, another for plugin setup and so on. This makes sense but makes it hard to follow what code is worker-related, what is not.

Proposal for improving (1):

1. Let's create two subdirectories under src/ - src/main and src/workers and src/shared - along the internal service boundary lines. So code in src/workers contains everything needed to run the worker threads, src/main for the main thread and src/shared for any code shared between the two (e.g. implementation of createServer, DB etc).
2. Both src/worker and src/main import from src/shared but from no other directory.
3. Within each subdir, we can further divide the code up as we already do - e.g. plugins, ingestion, etc.
4. Figure out a way to use composition rather than putting most not all queries into one db.ts file.