* Fix bug that <=> operator and in operator get wrong result
* Add some comment to get_result_for_null
* Add an new Binary Operator to replace is_safe_for_null for handleing '<=>' operator
* Add EQ_FOR_NULL to TExprOpcode
* Remove macro definition last backslash
* Add UserFunctionCache to cache UDF's library
This patch replace LibCache with UserFunctionCache. LibCache use HDFS
URL to identify a UDF's Library, and when BE process restart all of
downloaded library should be loaded another time. We use function id
corresponding to a library, and when process restart, all downloaded
libraries can be loaded without another downloading.
* update
