TagCache: Eliminate extra lock on cache load
TagCache: Eliminate extra lock on cache load

The original idiom, dating to when the cache was first implemented in
2011, used double-checked locking around getIfPresent on the cache. This
is basically treating the Cache as a bare HashMap with a single
process-wide lock, rather than taking advantage of the lock sharding
built into Cache. We can replace this with a simple call to Cache#get
instead.

While we're in there, add a constructor to EntryVal to make
initialization more concise.

Change-Id: I3ebc272d4882a0817188869b9892774c0a627dd8

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