This is a placeholder ticket to actually enable the use of the fixed-thread-pool for client handshaking following PM-1704. We didn't yet enable this feature because it depends on the completion of:
(1) Fully async streamable hello (PM-1705)
(2) Fully async authentication
(3) Finding an appropriate spot in the command path to move work to dedicated threads
For more details, see PM-1704 and ratika.gandhi's notes here:
https://docs.google.com/document/d/1oZG0390F2DIRoIRLLCaxFB_lyWBLdGH9oNUA1FuXPC4/edit