Druid Avatica Handler doesn't work correctly with search characters

[samarthjain]

Affected Version

Druid 0.18

The Druid Avatica metadata implementation, DruidMeta, doesn't work correctly when datasources have search characters like underscore( _ ) in their names. The general usage of the getColumns() api involves the caller escaping the search character like underscore _ by adding \ as escape character. If this escaping is not done, then the call to getColumns(some_datasource) will end up getting columns for some_datasource as well as somexdatasource which isn't correct.

[samarthjain]

Fix merged.