This fixes a plethora of "not indexed" errors when the document has not been indexed.
* Message handler throws NotIndexed if not overdue
* The message is put into backlog and tagged with backlog_path
* path2backlog[path] tracks backlog associated with document `path`
* The backlog is cleared when the index is merged
* backlog[0] is forced to run if it becomes overdue
textDocument/{declaration,definition,typeDefinition} return either LocationLink[] or Location[]
Add an initialization option client.linkSupport . When it is false, ccls will return Location[] disregarding client's linkSupport.
`struct LocationLink` does not include originSelectionRange as it is wasteful.
