company-backends: Add "to this command"
company-capf: Pass `sole' or 'finished' to exit-function, not `exact'
company-clang--start-process: Bind process-adaptive-read-buffering to nil

This speeds up completion on my system when a lot of candidates are returned, by
a factor of 2.5 (#228).

Inspired by https://github.com/Sarcasm/irony-mode/issues/131#issuecomment-60008803.