As OP_COMMANDREPLY can take ~48MB of output docs, and OP_REPLY (for commands) is limited to the 16MB document limit, commands that use output docs need to know when to stop appending documents to the response.

We should add a new method to ReplyBuilderInterface:

virtual std::size_t getRemainingSpaceInReply() const;

That returns how many bytes can be written to the message. Then we will need concrete implementations in CommandReplyBuilder (OP_COMMANDREPLY) and LegacyReplyBuilder (OP_REPLY)

cc: david.storch