Purge varnish items with "cdb_tablemetadata" channel on table update

Closes #296

Ensures that queries against CDB_TableMetadata for "last_updated"
(for example from Windshaft-CartoDB) are correctly invalidated.

You need to run bundle exec rake cartodb:db:load_functions to upgrade,
as already noted in the UPGRADE file.
Rake cartodb:db:load_varnish_invalidation_function
Merge remote-tracking branch 'origin/CDB-1206' into release/2.6.0
Merge branch 'master-cdb_tablemetadata-cachepurge' of github.com:strk/cartodb into release/2.6.0