Optimize memory usage per client

[zuiderkwast]

The client struct is quite large and valkey supports many connected clients.

• Fields only for replica clients can be stored in a wrapper which is allocated only for replica clients.
• Pubsub channels, pubsub patterns, etc. can use a similar wrapper maybe.
• Fields like resp can use a smaller integer type.
• Reordering fields so they can be packed more efficiently in the struct.

[virtualirfan]

Regarding reordering, has anyone done a cache line analysis to try and colocate fields that are often accessed together? Similarly to move apart fields that might be accessed to separate threads.

[zuiderkwast]

@virtualirfan Not that I'm aware of.

[madolson]

Similarly to move apart fields that might be accessed to separate threads.

Today clients are not concurrently accessed between multiple threads. (Although that will change soon, and we have done minimal analysis on that)