Don't lose concurrent modifications to %!conc_table

They were just fixed to not cause MVM_oop'es by #4496,
but as @nwc10++ pointed out, we might now silently lose them. So also
add a lock such that all thread's modifications are kept.

src/Perl6/Metamodel/Concretization.nqp

@@ -3,6 +3,8 @@ role Perl6::Metamodel::Concretization {
     has @!concretizations;
     has %!conc_table;
 
+    my $lock := NQPLock.new;
+
     method add_concretization($obj, $role, $concrete) {
         @!concretizations[+@!concretizations] := [$role, $concrete];
         nqp::scwbdisable();
@@ -36,13 +38,15 @@ role Perl6::Metamodel::Concretization {
     }
 
     method !rebuild_table() {
-        my %new_conc_table := nqp::clone(%!conc_table);
-        for @!concretizations {
-            %new_conc_table{~nqp::objectid(nqp::decont($_[0]))} := nqp::decont($_[1]);
-        }
-        nqp::scwbdisable();
-        %!conc_table := %new_conc_table;
-        nqp::scwbenable();
+        $lock.protect: {
+            my %new_conc_table := nqp::clone(%!conc_table);
+            for @!concretizations {
+                %new_conc_table{~nqp::objectid(nqp::decont($_[0]))} := nqp::decont($_[1]);
+            }
+            nqp::scwbdisable();
+            %!conc_table := %new_conc_table;
+            nqp::scwbenable();
+        };
     }
 
     # Returns a list where the first element is the number of roles found and the rest are actual type objects.