Investigate memory usage and an on-disk index

[fwcd]

Even on small projects the language server frequently uses several gigabytes of memory, especially when there are lots of symbols to be indexed (note that this includes the entire dependency tree!).

Since we probably do not query the database so frequently that we need it to be in memory, perhaps we could look into using an on-disk database such as SQLite? (This would additionally have the advantage of not requiring a reindex every time the project is opened, which would be in line with #337)

[D1360-64RC14]

Yeah, it needs some optimization.
It's impossible to use the VSCode plugin on my modest machine (8 GB of RAM). Doing some tiny work on a main function fills 3 GB of RAM in minutes.