Go back to using a file-backed SQLite database for tests

Using a memory database causes tests/test_build to intermittently
fail, because using the same pysqlite3 connection object from multiple
threads - as was done so that the threads shared the same memory database
- is not, in the end, thread safe. One thread will stomp on the transaction
state of other threads, resulting in errors from starting a new transaction
when another is already in progress, or trying to commit a transaction
that is not in progress.

To avoid a significant speed penalty, the session-scope fixture sets up
a database in the pytest temporary directory, which will typically be on
tmpfs. Time to complete all tests:

 memory backend:               38 seconds
 file on tmpfs:                40 seconds
 file on nvme ssd with btrfs: 137 seconds

MBSSQLAlchemy, which attempted to make the memory backend work, is removed.

Session hooks are installed on the Session class rather than on the
scoped_session instance - this works better when we're changing from
one database to another at test setup time.

module_build_service/common/config.py

@@ -41,6 +41,8 @@ class BaseConfiguration(object):
 
 class TestConfiguration(BaseConfiguration):
     LOG_LEVEL = "debug"
+    # This is just for initialization - we override this with a database in the
+    # pytest temporary directory
     SQLALCHEMY_DATABASE_URI = os.environ.get("DATABASE_URI", "sqlite:///:memory:")
     DEBUG = True
     MESSAGING = "in_memory"