Add a query caching capability to image mosaic

An image mosaic with multiple coverages (e.g. sentinel2) might end up performing the same query multiple times, especially in combination with a shared typename and multiple location properties.

This ticket adds an experimental capability to cache query results, for a configurable amount of time, and with a given maximum size, adding the following two properties in the datastore.properties file:

The max-age is expressed in milliseconds. As an experimental capability, it’s not officially documented on purpose, this ticket will serve as temporary docs while we decide if this is actually the best approach to handle the issue (or until we find the sponsorship for a better one).