C API: Differences in freeing result values between merge operator and compaction filter

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### Expected behavior
The C API frees the resulting `tmp_new_value` within `FullMergeV2` and `PartialMergeMulti`:
https://github.com/facebook/rocksdb/blob/e6f86cfb36c66bb912ec278007e4622e4529c567/db/c.cc#L366-L376

### Actual behavior
The C API does not free `c_new_value` in calls to `Filter`, however: https://github.com/facebook/rocksdb/blob/e6f86cfb36c66bb912ec278007e4622e4529c567/db/c.cc#L215-L228

Is it expected that users of the C API manage the memory for the `c_new_value`, or can `Filter` be updated to similarly free `c_new_value` after it is assigned to `new_value`?
I'd be happy to work on any required changes.

### Steps to reproduce the behavior