Summary:
`PartialMergeMulti` implementation is enough for Cassandra, and `PartialMerge` is not required. Implementing both will just duplicate the code.
As per https://github.com/facebook/rocksdb/blob/master/include/rocksdb/merge_operator.h#L130-L135 :
```
// The default implementation of PartialMergeMulti will use this function
// as a helper, for backward compatibility. Any successor class of
// MergeOperator should either implement PartialMerge or PartialMergeMulti,
// although implementing PartialMergeMulti is suggested as it is in general
// more effective to merge multiple operands at a time instead of two
// operands at a time.
```
Closes https://github.com/facebook/rocksdb/pull/2737
Reviewed By: scv119
Differential Revision: D5633073
Pulled By: sagar0
fbshipit-source-id: ef4fa102c22fec6a0175ed12f5c44c15afe3c8ca
132306fbf0