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.
When a replica uses the diskless-load swapdb approach, it backs up the old database,
then attempts to load a new one, and in case of failure, it restores the backup.
this means that modules with global out of keyspace data, must have an option to
subscribe to events and backup/restore/discard their global data too.
recently added more reads into that function, if a later read fails, i must
either free what's already allocated, or return the pointer so that the free
callback will release it.
looks like each platform implements long double differently (different bit count)
so we can't save them as binary, and we also want to avoid creating a new RDB
format version, so we save these are hex strings using "%La".
This commit includes a change in the arguments of ld2string to support this.
as well as tests for coverage and short reads.
coded by @guybe7
