dbapi: make arraysize a read/write property + default for fetchmany
dbapi: make arraysize a read/write property + default for fetchmany

So for example, given:

        cur.execute('SELECT * from auth_group')
        print(cur.fetchmany())
        for res in cur.fetchmany(size=10):
            print(res)

we now get back

        [(7, 'b')]
        (8, 'c')
        (23, 'b')
        (24, 'c')
        (167, 'b')

instead of
        [(7, 'b'), (8, 'c'), (23, 'b'), (24, 'c'), (167, 'b')]

Fixes #158

googleapis/python-spanner-django,googleapis/python-spanner-django