Make clobber interact appropriately with default values

[nmoroze]

I think the clobber flag to chip.set() currently works in an unintuitive way. The problem is it will always overwrite an "empty" value, even if it has been explicitly set. E.g.:

>>> import siliconcompiler
>>> chip = siliconcompiler.Chip('test')
>>> chip.set('option', 'remote', False)
>>> chip.get('option', 'remote')
False
>>> chip.set('option', 'remote', True, clobber=False)
>>> chip.get('option', 'remote')
True

I think it should not be considered correct behavior for the second chip.set() to override the value that the user has explicitly set. This behavior caused the server tests to fail in the first iteration of #1145, and I think a proper solution would be to fix this issue.

I think the fix for this would be to no longer set the 'value' field of parameters when the schema is first initialized, and rather just have chip.get() fall back to the defvalue transparently when no value is set. Then, sets with clobber=False will only take effect if the value field does not exist.

I'd like to refactor _search() at some point soon, after I finish splitting out schema access into a separate class. Once that happens, I can make this change and fix this issue.