gistfile1.txt

add two fields to append_entries_rpc:

read_index :: non_neg_integer()
echo :: boolean()

and one field to append_entries_reply:

read_index :: non_neg_integer()


All consistent queries are assigned a monotonic read index / token that is reset to 0 every time there is a leader change.

every append_entries_rpc includes the largest assigned read index and every append_entries_reply includes the highest read index seen
by each follower. The read index seen by each follower is stored in the "peer map" and can be used to work out the highest read index for which the leader has had a follower confirmation. Just like match_index tracks which log index each follower has confirmed they have seen.

Every time the read index "result" (the consensus calculation) is incremented by the follower all waiting readers can be served that 
have been assigned a lower or equal read index.

If the `echo` flag is set the follower knows to echo back an append_entries_reply immediately and not wait for the next disk write to complete