fixes #5130 - Added warning when apipie cache is missing or outdated

This adds one small check during startup when in development mode. The test
takes less than 0.050 seconds currently and check is apipie cache:

a) is present (if not - WARNING)

b) is outdated (if it is - WARNING)

Particularly state (b) is quite dangerous, I can see myself spending two hours
debugging a API code with outdated cache.

If you like this, I will make an issue for it.