Additionally the older defrag tests are using an obsolete way to check
if the defragger is suuported (the error no longer contains "DISABLED").
this doesn't usually makes a difference since these tests are completely
skipped if the allocator is not jemalloc, but that would fail if the
allocator is a jemalloc that doesn't support defrag.
Add a new set of defrag functions that take a defrag context and allow
defragmenting memory blocks and RedisModuleStrings.
Modules can register a defrag callback which will be invoked when the
defrag process handles globals.
Modules with custom data types can also register a datatype-specific
defrag callback which is invoked for keys that require defragmentation.
The callback and associated functions support both one-step and
multi-step options, depending on the complexity of the key as exposed by
the free_effort callback.
