[MsgPack] MsgPackDocument::readFromBlob now merges

The readFromBlob method can now be used to read MsgPack into a Document
that already contains something, merging the two. There is a new Merger
argument to readFromBlob, a callback function to resolve conflicts.

Differential Revision: https://reviews.llvm.org/D79671

Change-Id: Icf3e959217fe33cd907a41516c0386aef2847c0c