API : Revise ReplicatorProgress API

Based on the C-API discussion, we want to change ReplicationProgress to be like CBL-C:

 

typedef struct

{     uint64_t fractionComplete;     ///< Approximate completion; the value is between 0.0 - 1.0.     uint64_t documentCount;        ///< Number of documents transferred so far. }

CBLReplicatorProgress;