reenabled ghost tables (do not merge)

there are some things to review:
- how the ghost tables task is launched
- will it work with multiuser
- the user specs where not working

## test to be done

### before start
be sure ``ghost_tables_enabled`` is true for the user you are testing

### create table
- create a table using SQL API: "create table table_test (test int)"
- cartodbfy it using the sql api: select cdb_cartodbfytable('table_test')
- load the dashboard, wait 15 seconds
- load the dashboard again

the table should be there

### drop table
- load a table using the dashboard
- drop it using SQL API
- load the dashboard, wait 15 seconds
- load the dashboard again

the table should *not* be there

### rename table
- load a table using the dashboard
- rename it using SQL API
- load the dashboard, wait 15 seconds
- load the dashboard again

the table should *not* be there

create these tests using a MU account

ideally these tests can be performed using the SQL API and the REST API so they can be automated using python/bash
I'm  a bit afraid that always checking this could be a performance hit, as the check will be executed on every API call (we're crossing at minimum two queries, user real tables and metadata tables).

We can just deploy it and watch the stats, or maybe have a user flag with the timestamp of last ghost tables check, so it doesn'checks and adds again if not past X time (probably just 1h would be enough).