buffer: track asynchronous buffers and don't release them until unref

synchronous buffers are read instantly and we can release them, but asynchronous ones have to be locked until they are unref'd from .current to avoid reading from a buffer after .release()

src/protocols/core/Shm.cpp

@@ -49,6 +49,10 @@ eBufferType CWLSHMBuffer::type() {
     return BUFFER_TYPE_SHM;
 }
 
+bool CWLSHMBuffer::isSynchronous() {
+    return true;
+}
+
 SSHMAttrs CWLSHMBuffer::shm() {
     SSHMAttrs attrs;
     attrs.success = true;