[Fixes #11512] improves cache size calculation in ActiveSupport::Cache::MemoryStore

Previously, the cache size of `ActiveSupport::Cache::MemoryStore` was calculated
as the sum of the size of its entries, ignoring the size of keys and any data
structure overhead. This could lead to the calculated cache size sometimes being
10-100x smaller than the memory used, e.g., in the case of small values.

The size of a key/entry pair is now calculated via `#cached_size`:

    def cached_size(key, entry)
      key.to_s.bytesize + entry.size + PER_ENTRY_OVERHEAD
    end

The value of `PER_ENTRY_OVERHEAD` is 240 bytes based on an [empirical
estimation](https://gist.github.com/ssimeonov/6047200) for 64-bit MRI on
1.9.3 and 2.0.

Fixes GH#11512 https://github.com/rails/rails/issues/11512

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