Add function parse_before_bind() which is used in Load balance mode.

Add function parse_before_bind() which is used in Load balance mode.
Add function overwrite_map_for_deallocate() which is used to overwrite
where_ot_send map by a specified query context.
Add length and contents arguments to private functions that process message.
Remove pending_function variable.
Change ReadyForQuery() so that it does not destroy a query context in
extended query protocol.
Refactor following functions.
- Parse()
- Execute()
- ProcessFrontendResponse()
Add private functions that process message.
- Bind()
- Describe()
- Close()
- FunctionCall3()
- ParseComplete()
- BindComplete()
- CloseComplete()
- CommandComplete()
- ErrorResponse3()