The flatbuffers `Table::ClearField` method is dangerous

[bsilver8192]

We patch flatbuffers to add a Table::ClearField method. This method modifies an existing vtable. I think this vtable may be shared with other tables (in the same buffer), which means it will clear the same field for them too.

I think this is the reason upstream doesn't have a GetMutableVTable, because there aren't really any modifications to a vtable which are safe at this point in its lifecycle.

Presumably there's some way to find/create a vtable with the field unset and swap to that, but I have not yet dug in enough to find it.

[AustinSchuh]

Correct, you need to be decently sure it isn't shared to clear it.

The static flatbuffer API is generally a lot nicer, and shouldn't have these problems. Which says maybe we should remove it from the normal API.