Finisher: use complete() not finish() and delete

Signed-off-by: Samuel Just <sam.just@inktank.com>
FileStore: use complete() instead of finish() and delete

Signed-off-by: Samuel Just <sam.just@inktank.com>
src/*: make Context::finish private and switch all users to use complete

Signed-off-by: Samuel Just <sam.just@inktank.com>