Keep configurable number of messages per context key, regardless of expiration time

[jlyons210]

Enhancement:
Currently, the bot discards conversation context after 15 minutes. Since chat conversations are sometimes very asynchronous, it would be nice to have a configurable number of messages per context key (conversation thread) that are maintained indefinitely.

Say we have a conversation and there are 18 messages. If the context timeout is 15 minutes and a minimum retained messages is 5, the 5 most recent messages would be retained until the conversation picked up again.

This will also be implemented in envoy-ai.