With this fix, module data type registration will fail if the load or save callbacks are not defined, or the optional aux load and save callbacks are not either both defined or both missing.
This adds a copy callback for module data types, in order to make
modules compatible with the new COPY command.
The callback is optional and COPY will fail for keys with data types
that do not implement it.
With the previous API, a NULL return value was ambiguous and could
represent either an old value of NULL or an error condition. The new API
returns a status code and allows the old value to be returned
by-reference.
This commit also includes test coverage based on
tests/modules/datatype.c which did not exist at the time of the original
commit.
Add two new functions that leverage the RedisModuleDataType mechanism
for RDB serialization/deserialization and make it possible to use it
to/from arbitrary strings:
* RM_SaveDataTypeToString()
* RM_LoadDataTypeFromString()
