Only close db connection when disconnecting, keep the connection object
Create DB Connection wrapper and DB\Adapter* classes
Move db prefix handling to Connection wrapper
Move lastInsertId to adapter classes
Move prepared query cache handling to Connection wrapper
Move limit/offset handling to Connection wrapper
Move query statement fixup handling to Connection wrapper
Move insertIfNotExist to Connection wrapper

Real implementation is in DB\Adapter* classes
Simplify OC_DB transaction functions
Use $connection for schema changes
Cleanup: remove code for multiple database backends
Group database connection and schema function
Move building error string to connection
Correct namespace of OC_ classes
Fix errors from unit tests