for better cross version compat - continue to use "SolidHandle" as the serialized name for NodeHandle
not sure I fully understand the implications of this.
If we serialized these with the NodeHandle name, is the problem that old pipeline snapshots would not be possible to deserialize? If that's the case, will this special case serializer need to be around forever?
So  handles the loading old SolidHandle strings in to NodeHandle
the problem this diff addresses is *old* code trying to load the something serialized by the *new* code. Since NodeHandle doesn't exist in old, it will fail if it trys to load something serialized by new.
This shouldn't really happen ideally, I think it will be reasonable for us to expect dagit is as new as the newest user code deployment for example
but there are some current set-ups we have that don't abide by this so im just being extra generous with the back compat.
I think something like  we will keep around for a long time - but this serializer we can potentially remove if we better enforce the scheme described above