optimize quorum read

[FANNG1]

Now quorum read go through raft and will apply to every follower's store.
1, every follower will apply a read and get the data from store, no need?
2, is it necessary to go though raft? could the follower get the newest commitIndex of leader, and wait util the log with the commitIndex applied to follower, then get the data from the store ?

[mitake]

After this PR, a follower skips log applying if the log doesn't have side effect (e.g. Range, read operations in Txn): #5689

Also, this PR implemented readIndex mechanism: #6212
With the change, a leader can execute read only requests without appending log entries.

[xiang90]

already implemented in 3.1. so closing.