XDCR: Remove duplicate data in P2P data objects

P2P data transfer structs have actual payload and compressed form as well, thereby duplicating the data.

Ideally after serializing/deserialising/compressing/decompressing the original data can be discarded. 

type ReplicationPayload struct {

    mtx               sync.RWMutex

    payload           *BucketVBMPayloadType

    PayloadCompressed []byte

    ReplicationSpecId string

    SourceBucketName  string

    InternalSpecId    string

}

The payload is the serialized which is then compressed to store in PayloadCompressed