Reference lookup perf improvement and table schema versioning

[sineed]

depends on elixir-tools/gen_lsp#37

The fix was simple to replace :dets.select/2 in favor of :dets.lookup/2. But it required a change in how references are stored. So the mechanism to clear dets table and trigger recompilation was added. It should recompile initially as the current table schema version is nil and every time the schema version value (for example, @reference_table_schema_version_value) is changed. I covered both symbol and reference tables.

This will also conflict with the hovering PR as I moved symbol logic to its own GenServer there but it should be not a big deal to resolve here or there

[mhanberg]

I switched the symbol/ref tables to use sqlite3 instead, so now lookups are very fast.