branchcache: change the _delayed flag to an explicit `_dirty` flag
This is more consistent with the logic we use for other object and it open the way to a clearer management of the cache state.
Now, cache are created clean, cache update mark them dirty, writing them on
disk mark them clean again.
{
// Enforcing
"eqeqeq" : true, // true: Require triple equals (===) for comparison
"forin" : true, // true: Require filtering for..in loops with obj.hasOwnProperty()
"freeze" : true, // true: prohibits overwriting prototypes of native objects such as Array, Date etc.
"nonbsp" : true, // true: Prohibit "non-breaking whitespace" characters.
"undef" : true, // true: Require all non-global variables to be declared (prevents global leaks)
// Environments
"browser" : true // Web Browser (window, document, etc)
}