SQLiteStorageEngine can return stale data in rare cases

[borrrden]

Reproduction steps:

This scenario requires three thread to all fall into a certain sequence:

1. Thread A starts a query that takes a long time (implicitly begins read transaction on read connection)
2. Thread B inserts a document (using write connection)
3. Thread C retrieves the documents (using read connection)
4. Thread A finishes

Step 3 will show the document as missing, despite it existing. This is because Thread A has locked the read connection on a stale state with its implicit transaction. There are two ways to fix this:

1. Serialize queries as well as writes
2. Let each thread have a separate read connection to the database

Option 2 would be more in line with what iOS does