detect row-switches for version_id_col

First, version_id_col doesn't respect row switches. Solution is to fetch the old version from the row_switch state.

Second issue is similiar, but more like an "offline row-switch" where version=1. Consider two concurrent transactions:

# tx1
x = db.query(X).get(1) # version = 1
db.delete(x)
db.commit()
# ...
db.add(X(data='"offline" row-switch'))
db.commit() # version = 1

# tx2 concurrent with above
x = db2.query(X).get(1) # version = 1
x.attr = 'I overwrote row-switch!'
db.commit() # version = 2

You may think that it's just an edge case for version=1, but that's probably the most common version number!

The solution in this case is to use version identifiers that aren't local to the object, e.g. UUIDs.

I have a patch that allows supplying a version_id_generator function, defaulting to an integer increment.

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