`TreeManager` is inherited by `InsertManager` but `where` on
`InsertManager` doesn't work.
And also, remove `@ctx` since the instance variable is used only for the
`SelectManager`.
Currently `SelectManager` takes a table (it was happened at 80ad95b),
but other managers does not, even though it is required to generate a
query.
I think that all tree managers' initializer could take a table, like
`SelectManager` already does.
