Added AddOrUpdate method to lru

[gkoutsou]

It behaves similar to the standard Add method with the difference that it can also take an update method that will be performed in case the key already exists.

For example:

value1 := []string{"some_value"}
value2 := []string{"some_value_2"}

l.Add(1, value1)
l.AddOrUpdate(1, value2, func(v1, v2 interface{}) interface{} {
	slice1 := v1.([]string)
	slice2 := v2.([]string)
	return append(slice1, slice2...)
})

Then l.Get(1) will result to []string{"some_value", "some_value_2"}

[hashicorp-cla]

All committers have signed the CLA.