worker: per-tenant cache shard + tenant-scoped read/serve path

[somfornot]

Shard worker cache state by tenant (HashMap<TenantId, TenantShard> with a cache_root/<tenant>/ subtree) under a single global eviction budget (shard the eviction policy, not the budget). Thread the tenant past admission into serve/block resolution on the read path.

Part of #555. Design: #554.

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