Use SingletonThreadPool as the default connection pool to guaranty connection isolation between threads

SQLite cannot handle connections shared across thread boundaries so it's safer to enforce this constraint at the SQLAlchemy pool level.

Otherwise one could get low level errors such as:

ProgrammingError: (ProgrammingError) SQLite objects created in a thread can only be used in that same thread.The object was created in thread id 140735148290432 and this is thread id 4668608512 None None

when two threads access the model concurrently using the save session maker pool.