Track respawned workers, for possibly forthcoming SIGQUIT.
Add shared in-memory cluster cache.

Enable usage of the cluster cache by passing createCache:true in the
cluster config object. Cluster workers can access the cache using the
cluster.cache.get/set operations. The cluster master receives get/set
messages from the workers, accesses the cache, and responds using
messages to the workers.

Currently the master’s cache is backed by the node-cache package with a
TTL of 60s (configurable via config.cacheTTL) and a purge interval of
30s (configurable via config.cacheCheckPeriod). Cache size is unbounded,
but the low TTL hopefully reduces this risk.