When the current file is X.cc, there might be multiple X.h.
Use a heuristic to find the best X.h.
Vote for each interesting symbol's definitions (for header) or
declarations (for non-header). Select the file with the most votes.
If `file_id2cnt` is empty, use a simpler heuristic.
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
* WorkingFiles::files : vector -> unordered_map
* Add timestamp to WorkingFile
* Rename "comp-preload" thread to "preamble"
* Rename CompletionManager to SemaManager as it is used by "diag" "comp" "preamble"
* Rename clang_complete.* to sema_manager.*
* Merge SemaManager::{preloads,sessions}
* Add initialization option session.maxNum
* In DiagnosticMain, if an included file was modified, cancel the DiagTask and create a PreambleTask instead. The task sets `from_diag` so as to trigger immediate DiagTask after the preamble is built.
