Re-name metastore to dkan_metastore

[dafeder]

In order to get DKAN to play nice with the rest of the drupal.org ecosystem, its submodules really need to be namespaced correctly. Rename metastore dkan_metastore (and rename its submodules similarly). Rename it and update references everywhere, but keep all functionality the same. Do not change service names, just namespaces.

We'd still like to merge it into dkan and common modules (#4413) but for now this will help resolve immediate issues.