refs: Prohibit IDs consisting of all zero bytes

Container and object IDs are SHA-256 checksums of their contents. It is
almost impossible to find data that corresponds to a zero identifier.
Setting 32 zeros rather indicates a client error. Reserving a zero value
will also make application development easier: corresponding type will
have a reserved invalid value within itself. Even if the unimaginable
and some structure will actually have a zero hash - so be it, the
problem is negligible for the final benefit.

Note that zero `OwnerID` is already invalid at least due to the non-zero
prefix. Therefore does not need clarification.

Signed-off-by: Leonard Lyubich <leonard@morphbits.io>

proto-docs/refs.md

@@ -71,7 +71,8 @@ content-addressed.
 
 `ContainerID` is a 32 byte long
 [SHA256](https://csrc.nist.gov/publications/detail/fips/180/4/final) hash of
-stable-marshalled container message.
+stable-marshalled container message. ID consisting of all zero bytes is
+reserved for undefined value and must not be specified as a field.
 
 String presentation is a
 [base58](https://tools.ietf.org/html/draft-msporny-base58-02) encoded string.
@@ -97,7 +98,8 @@ It means `ObjectID` will change if the `header` or the `payload` changes.
 `ObjectID` is a 32 byte long
 [SHA256](https://csrc.nist.gov/publications/detail/fips/180/4/final) hash of
 the object's `header` field, which, in it's turn, contains the hash of the object's
-payload.
+payload. ID consisting of all zero bytes is reserved for undefined value and
+must not be specified as a field.
 
 String presentation is a
 [base58](https://tools.ietf.org/html/draft-msporny-base58-02) encoded string.

refs/types.proto

@@ -22,7 +22,8 @@ message Address {
 // `ObjectID` is a 32 byte long
 // [SHA256](https://csrc.nist.gov/publications/detail/fips/180/4/final) hash of
 // the object's `header` field, which, in it's turn, contains the hash of the object's
-// payload.
+// payload. ID consisting of all zero bytes is reserved for undefined value and
+// must not be specified as a field.
 //
 // String presentation is a
 // [base58](https://tools.ietf.org/html/draft-msporny-base58-02) encoded string.
@@ -42,7 +43,8 @@ message ObjectID {
 //
 // `ContainerID` is a 32 byte long
 // [SHA256](https://csrc.nist.gov/publications/detail/fips/180/4/final) hash of
-// stable-marshalled container message.
+// stable-marshalled container message. ID consisting of all zero bytes is
+// reserved for undefined value and must not be specified as a field.
 //
 // String presentation is a
 // [base58](https://tools.ietf.org/html/draft-msporny-base58-02) encoded string.