[server] Initialization status

[Simn]

The compilation server currently has a single initialized flag which governs the class path exploration. This works fine for single contexts, but misses information if there are multiple contexts involved.

As an example, starting the server with a Hello World program and then adding -lib flixel to the build will cause completion requests to miss the flixel packages and types.

We should instead track initialization status per-signature.

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A related problem is the change of class paths without changing the signature. This is a rare case which only occurs by adding or removing explicit -cp arguments (because -lib comes with -D which changes signature). We already detect changed class paths, so this is a matter of unsetting the initialization status.