InMemoryStore: improve removeAllByIndexValues over natural key index

The method `removeAllByIndexValues` in KVStore is to delete all the objects which have certain values in the given index.
However, in the current implementation of `InMemoryStore`, when the given index is the natural key index, there is no special handling for it and a linear search over all the task data is performed.

We can improve it by deleting the natural keys directly from the internal hashmap.