fix(protocol): release Data charge to its owning session

Data charged against a ProtocolSession kept only the charge amount, so
data_destroy released it from whatever session was thread-locally bound
at destroy time. Destroying a received Data on another thread, after the
session was unbound, or while a different session was bound leaked the
originating session's budget and underflowed the other's.

Add Data.owner, set it whenever protocol_receive_data_limited charges a
session, and have data_destroy release against that owner directly via
the newly-exported protocol_release_memory_for_session. Uncharged Data
(owner NULL) keeps the previous bound-session fallback.

Add a unit test proving a Data acquired on session A is released to A
even when unrelated session B is bound at destroy time.

src/shared/protocol.c

@@ -40,7 +40,7 @@ static bool protocol_reserve_memory(ProtocolSession* session, size_t charge) {
   }
 }
 
-static void protocol_release_memory_for_session(ProtocolSession* session, size_t charge) {
+void protocol_release_memory_for_session(ProtocolSession* session, size_t charge) {
   unsigned long long allocated = atomic_load(&session->total_allocated_bytes);
   while (true) {
     unsigned long long remaining = (unsigned long long)charge >= allocated ? 0 : allocated - charge;
@@ -573,6 +573,7 @@ Data* protocol_receive_data_limited(ProtocolSession* session, unsigned long long
     return NULL;
   }
   result->protocol_charge = allocation_size;
+  result->owner = session;
   return result;
 }