pw_kvs: Transcation ID and key descriptor refactor

- Switch from per-key versions to a monotonically increasing KVS-global
  transcation ID.
- Move KeyDescriptor and the hash function to their own files.

Change-Id: I05287137579d4fe2d72c6e176969d46006c2aae6
diff --git a/pw_kvs/BUILD b/pw_kvs/BUILD
index 50601da..6adb71a 100644
--- a/pw_kvs/BUILD
+++ b/pw_kvs/BUILD
@@ -30,6 +30,8 @@
         "entry.cc",
         "flash_memory.cc",
         "key_value_store.cc",
+        "public/pw_kvs/internal/hash.h",
+        "public/pw_kvs/internal/key_descriptor.h",
         "public/pw_kvs/internal/sector_descriptor.h",
         "pw_kvs_private/entry.h",
         "pw_kvs_private/macros.h",