Add last modified time for metadata

This patch improves performance during development and does not affect production performance.

Currently, enabling the metadata cache during development gives a remarkable performance boost, but it requires that you manually flush the metadata cache when the metadata has been changed. When running console commands such as ``console orm:schema-tool:update`` (see #979).

With this change, metadata entries fetched from cache are checked for freshness using a quick last modified check. This is much faster than loading the actual metadata using the metadata driver. This allows you to enable the metadata cache during development without the need for manually flushing the metadata cache.

This PR is part 1 of 2. Part 2 is for the doctrine/common repository.

If this PR is accepted, it allows us to optimize the auto-generate proxy classes feature by comparing the last modified time for the metadata with the modified timestamp of the generated file.