Add startup check for large read_ahead_kb setting

A well known production tuning recommendation is to dial back the read_ahead_kb Linux setting from the default of 4096 in most distributions as this can cause high IO usage and cache churn on read-intensive workloads.

We should add a startup warning to detect when a high read_ahead_kb is used and recommend the user to dial back to a reasonable value, which varies according to the workload but a general guideline is to tune this to 8kb.