This adds an asynchronous method to lock the startup files using the same
lock that we use during normal startup.
The profile service then uses this lock to gate access to the profiles.ini
files adding a method to async flush the entire database or in the case
that the on-disk database has changed a way to mergwe in some properties
about the current profile into the on-disk version.
Differential Revision: https://phabricator.services.mozilla.com/D222662
75cb8ade57