Add bundle reader abstraction

Created by: efritz

Create an abstraction between the bundle manager database and the storage layer. This makes a solid interface that we can use to later abstract out the storage layer if we find something that works better.

The worker will have a symmetric interface for writing to a storage layer. The serialization package was also added to help us deal with migration between serialized data versions.

Major changes:

• Wrote the sqliteReader and the serialization package in internal/codeintel
• The bundle manager's Database is now databaseImpl, and Database is now an interface
• The databaseImpl SQLite-specific code now delegates to the new sqliteReader

This partially implements https://github.com/sourcegraph/sourcegraph/issues/9965.