Use 2 iterations for local password hashing (#1423)

* Add HashPurpose parameter to HashPasswordAsync

* Use 2 iterations for local password hashing

* Force logout if user has old keyHash stored

* Revert "Force logout if user has old keyHash stored"

This reverts commit 497d4928fa.

* Add backwards compatability with existing keyHash

src/Core/Enums/HashPurpose.cs

@@ -0,0 +1,8 @@
+namespace Bit.Core.Enums
+{
+    public enum HashPurpose : byte
+    {
+        ServerAuthorization = 1,
+        LocalAuthorization = 2,
+    }
+}