Autocompletion improvements

[snuyanzin]

The idea is to improve existing autocompletion with several points

1. No need for autocompletion if the cursor is inside single quoted string or a comment
2. Schemas should be autocompleted
3. It would make sense to complete first only schema or table name and then table or column separately rather than suggest e.g. table_name + "." + column_name for all possibilities.
4. If the cursor is inside sql identifier quoted area then only schemas/table/column makes sense for completion
5. If a schema/table/column name contains whitespace it should be completed with required sql identifiers
6. Update tests to simplify junit tests of all cases mentioned above

[snuyanzin]

Merged as 32a915c