Followup items from #758 Async IO threads

[madolson]

List of pending items from #758, all of these are future enhancements, and not pending issues that need to be merged before Valkey 8 (but they could be).

• [ ] Remove err_clean in tls
• [ ] Offload replica and master traffic to IO threads.
• [ ] Optimize Client struct memory foot print.
• [ ] Parsing offload to parse multiple commands.
• [ ] Extend IO free offload to dbDelete - handle defrag
• [ ] lazy free to use IO threads?
• [ ] Offload TLS negotiation.
• [ ] Move all client’s fields accessed by the IO threads to a sub-struct.
• [ ] Allow dynamic modification of IO threads num.

[uriyage]

• [ ] Avoid recalculating slots when the I/O thread has already done so

[zuiderkwast]

• [ ] Offload PING command