[fix] Unload fails when a query has backslash
[fix] Unload fails when a query has backslash

Unload fails with syntax error when a query has
backslash in it.

Example query with backslash
```
select
    replace(column, '\\','\\\\')
from
    table
;
```

Generated unload query
```
unload ('select replace(event, \'\\\',\'\\\\\') from global_log.app_pv_logs where log_time between \'2018-01-01 00:00:00\' and \'2018-12-31 23:59:59\' ;')
to 's3'
credentials ''
delimiter '\t'
;
```

Error message
```
ERROR:  syntax error at or near "\"
LINE 1: unload ('select replace(event, \'\\\',\'\\\\\') from global_l...
```

`replace(event, \'\\\',\'\\\\\')` should be
`replace(event,\'\\\\\',\'\\\\\\\\\')`.

aamine/bricolage,aamine/bricolage