Increase #frontend threads for large node instances

Currently memcached creates frontend threads based on the number of CPU cores - see get_number_of_worker_threads(). This is currently 3/4 of the number of CPU cores for systems with at least 5 cores).

However, this was set when systems had smaller CPU counts. Systems with high core counts are more and more common, and limiting the front-end thread count to 75% of the cores leaves a signficnant amount of resource unused for larger systems.

Increase to a higher percentage on larger systems. Proposal is to create #CPUs * 7/8 (87.5%) for systems with 16 or more cores.