Improving successful TLS handshake throughput with concurrent control

When there are lots of clients trying to re-establish sessions, there might be lots of half finished handshake timed out, and those failed ones keep reconnecting to another server and restarting the handshake from beginning again, which caused herd effect.
 
And the number of total ZK sessions could be supported within session timeout are impacted a lot after enabling TLS.
 
To improve the throughput, we added the TLS concurrent control to reduce the herd effect, and from out benchmark this doubled the sessions we could support within session timeout.