Refactoring

[omegaga]

We need to do some refactoring. Some are listed below:

1. Message handlers should be wrapped in functions. DONE
2. When duplicate prepare statement name occurs, plan cache should reject and return error to frontend
3. Plan cache and portals should be stored in PacketManager, instead of a thread_local variable2
4. Add query_type to plan cache so we don't need to read it again in BIND message
5. variable name row_affected is misleading, change to a more appropriate name
6. We should use a generic value type in BIND message, instead of converting to string and back into data types
7. Check if we do unnecessary copying for strings, vectors, etc.
8. Replace ASSERT to error handling code to make our code more robust
9. Other 'TODO's in code