Move inflate/deflate in FileSource to separate thread

[kkaefer]

We're currently calling inflate/deflate synchronously in the SQLiteCache. Instead, we should reserve this cache for SQLite operations exclusively and do other time-consuming tasks in a separate thread.

Once this is asynchronous, we can no longer guarantee an order of get/put operations, since compress tasks might run in parallel and can take more or less time compared to other cache tasks.

[jfirebaugh]

What's the rationale?

[kkaefer]

It takes some time to decompress/compress the data. During this time, we can't do I/O with the the database.

[jfirebaugh]

If we do #3722, we can't have asynchronicity within the database code. Instead, we should do #3886.