PARQUET-7: [thrift] avoid thrift amender if all fields are optional.

No perf tests... I did some rudimentary testing which seems to bear out the idea but nothing scientific yet.
why expire? It's possible for this to change during the lifetime of the jvm?
Or you want to avoid the map growing too big? Isn't a parquet schema inherently tied to a single thrift schema? Do we load more than one schema in the same jvm? If it's just one schema then this shouldn't grow without bound right?