feat(sharding): Cache shard mappings

This patch modifies sharding.Catalog so that it caches the mapping
between (project, queue) ===> shard. This reduces latency for
all data plane operations when sharding is enabled.

Change-Id: I54714c4e6c0ed78ac302ce86aed9bb72b200052b
Partially-Implements: blueprint storage-sharding
Merge "feat(sharding): Cache shard mappings"