signals: Rebuild stale global cache entries automatically

[tekacs]

When rebuilding with dx serve --hot-patch, changing the type of the value held in a GlobalSignal currently results in a panic, due to a downcast failure.

This patch causes the runtime to evict the existing value with a warning when that happens, rather than panicking.

I considered making the type part of the key, but this feels gentler, since this is a rare occurrence.