Finished implementation of sync_exchange.

Added new message structures: DocInfo, AllSentInfoRequest and AllSentInfoResponse.

Updated AllSentResponse message struct and added some comments.

Some doc fixes.

Added helper utilities for new message structures.

Added tests for new message structures.

Updated message structures.

Updated tests.

Completed sync_exchange method.

Added server side logic to handle intermediate ping ack. Fixed bugs in sync_exchange. Finally works.