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