a Caveat can now contain multiple rules

This allows us to implement "OR" for caveats: one of the provided rules
has to succeed

schema.proto

@@ -25,7 +25,7 @@ message Block {
   repeated string symbols = 2;
   repeated Fact   facts = 3;
   repeated Rule   rules = 4;
-  repeated Rule   caveats = 5;
+  repeated Caveat caveats = 5;
   optional string context = 6;
 }
 
@@ -39,6 +39,10 @@ message Rule {
   repeated Constraint constraints = 3;
 }
 
+message Caveat {
+  repeated Rule queries = 1;
+}
+
 message Predicate {
   required uint64 name = 1;
   repeated ID ids = 2;