DataGrid column instances are shared across multiple instances of a

DataGrid. This would be fine, except we were storing important state on

the column instances, which preventing DataGrids from being reliable in

a multi-threaded configuration (such as Apache's mod_worker or mod_wsgi

daemon mode).
DataGrid now creates wrappers around a Column, using the new

StatefulColumn class. These exist for the lifetime of a DataGrid

instance, and stores state about the associated column. It also wraps

all attribute and function access for the column, forwarding all

requests for attributes not otherwise stored on StatefulColumn.
Column functions now all take a 'state' parameter, which StatefulColumn

will automatically pass when wrapping a function. This parameter is used

for any state-related access.
A couple things were also cleaned up to help with this. Some work that

the Columns were doing (such as constructing Template objects on a

DataGrid) have been moved to the DataGrid.

Updated Review Board for these changes, and tested adding every column,

sorting many columns, and rearranging columns. Everything worked exactly

as it did before.
Layouts were also properly saved and loaded.
Unit tests pass.