Store default value text instead of default value expression in metastore

Currently for default value an expression is generated and serialized to store in metastore. It should be improved to rather serialize the default value itself instead of expression and store that in metastore. This will have the following benefits:

• It will make metastore schema upgrade safe. e.g. if a UDF function name is changed hive wouldn't be able to parse back the expression for this UDF which was serialized in earlier version.
• It will make metastore schema for default constraint hive agnostic. Other databases would be able to use the value as it is.