This fixes the following problem found during testing:
```
ALTER EXTENSION cartodb UPDATE TO '0.8.0';
ERROR: cannot change return type of existing function
HINT: Use DROP FUNCTION cdb_usertables(text) first.
```
Roles are not created anymore, previously private functions for table
information extraction (CDB_UserTables, CDB_TableIndexes, CDB_ColumnNames,
CDB_ColumnType) will now be callable by anyone while only returning
information about tables over which the calling user has SELECT privilege.
Closes#36
We'll bet on no old signature being left around.
Presence of an old signature would break at runtime
with a message like:
ERROR: function CDB_UserTables() is not unique
Closes#25
NOTE: also makes CDB_TableMetadata triggers still functional
when user quota and varnish invalidation functions are missing
from the database (currently installed by cartodb ruby)
