pw_kvs: Don't write key values that don't change

Do not write incoming key vales that match the already-stored entry. The
check it done by comparing entry state, value length, checksum, and
actual value.

Change-Id: Ibf2f1145cf59144b832af8ae8b2e5580e91091a1
diff --git a/pw_kvs/BUILD b/pw_kvs/BUILD
index dbbddd3..15899ad 100644
--- a/pw_kvs/BUILD
+++ b/pw_kvs/BUILD
@@ -52,6 +52,7 @@
     ],
     includes = ["public"],
     deps = [
+        "//pw_assert",
         "//pw_checksum",
         "//pw_containers",
         "//pw_log",