storage: tweak raftEntryCache allocation
storage: tweak raftEntryCache allocation

Using cacheEntryKey and raftpb.Entry as values to OrderedCache implied
an allocation for storing the values in interfaces. There was an
additional allocation for the cache.Entry. These 3 allocations have been
reduced to 1 using the same approach as TimestampCache: an unnamed
struct composed of the key, value and cache.Entry.

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