Use historical nodes to host shared cache

There is an idea that instead of memcached, historical nodes themselves can be used to host *shared*, not only their local cache. Since there are a lot of them, only a small fraction of each historical node's memory can be devoted to the shared cache.

Such colocation can also simplify Druid setup because no separate fleet of memcached nodes would be required.

No files, tests, or entry points are named. Start by tracing the existing memcached integration and historical-node configuration, then determine the design and operational requirements for colocating shared cache memory. Done means an agreed implementation scope and validation plan for using historical nodes instead of a separate memcached fleet.

Written by the indexing model from the issue text.