Some docs for CandidateRepository
Better thread safety in IdentifierCompleter

Everything that needs access to filetype_map_ has been split into a new class
called IdentifierDatabase. This class is thread-safe. Previously, multiple
threads could access filetype_map_ at the same time, some reading things from
it, others writing to it. WTF was I doing? My best guess is that I introduced
this vile stupidity when I added the second thread to IdentifierCompleter;
previously it was impossible for multiple threads to stomp on filetype_map_
because only one thread ever accessed it. I changed that some time ago and
forgot to protect filetype_map_.

Idiot.