Currently appending output documents is a no-op on a LegacyReplyBuilder.

To use output documents in find and getMore commands we will need to implement it as such:

The LegacyReplyBuilder will stash the command reply internally after downconverting it instead of immediately appending it to the message. Then when done() is called, it will downconvert the outputdocs to an inner field of the command reply (the format of find and getMore responses is detailed here: https://docs.google.com/document/d/1RDlyPRSr8zgl3vNTb56wOy7RzVooBytR5DqlIt8fG4w/edit#).

A complication arises due to the slightly different format of find and getMore reponses ("firstBatch" vs "nextBatch"). We will probably need to do something ugly here eventually, but the initial version can just always downconvert the output docs to the "firstBatch" format until we start modifying the find/getMore commands to use the replyBuilder directly.

cc: david.storch