Readonly properties

[dktapps]

Before this, readonly was silently ignored, so properties would be treated the same as normal properties.

This change enforces write-once and no-unset policies for readonly properties.

It also permits lockless reads of locally cached property values of readonly properties, since these can't be changed by another thread after initialization. This offers a performance benefit for objects whose properties are frequently accessed by multiple threads by reducing lock contention.

Currently this only covers stuff that is kept in the local cache, such as thread-safe objects, closures, sockets and strings. The cache could be extended to more stuff to permit this optimisation to apply to other types of properties, but that's a task for another time.