Don't allow expired contexts to retrieve topo connection
Don't allow expired contexts to retrieve topo connection

Once a context is expired, we shouldn't return a valid cell anymore,
even if it's the global cell.

Local cells would fail in this case as well, so this ensures we match
that behavior.

Signed-off-by: Dirkjan Bussink <d69eedf9386eafb922b5983901ebb19141493da3@gmail.com>

mahak/vitess,mahak/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,vitessio/vitess,vitessio/vitess,mahak/vitess,vitessio/vitess,vitessio/vitess,vitessio/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,mahak/vitess,vitessio/vitess