Defer constructing actor until actor actually waits

I think this potentially could be semantically different since it will change the number of times a copy constructor gets called in some cases, but I hope we don't rely on this behavior.

The issue names no files, tests, or entry points. Start by locating the C++ actor construction and wait implementation, then investigate the copy-constructor behavior mentioned in the issue. Done means construction is deferred until the actor waits without unintended semantic changes, with tests covering the affected behavior.

Written by the indexing model from the issue text.