conversation: link file before announcing commit

A sent file was linked after the commit was announced
to the client and to other devices, which was causing various
race conditions.

We add a new onCommit callback allowing to take action after a commit
is made and before it's announced to clients and other devices,
and we use this callback to link the file.

Change-Id: If5c64271b08a71749e5bf84426c5a6ecb9806ea6