[IMP] mail: add missing index
[IMP] mail: add missing index

Without this index, deletion of mail.message records takes too much time

stephen144/odoo,sysadminmatmoz/OCB,microcom/odoo,Elico-Corp/odoo_OCB,sysadminmatmoz/OCB,microcom/odoo,sysadminmatmoz/OCB,syci/OCB,stephen144/odoo,bplancher/odoo,dfang/odoo,syci/OCB,bplancher/odoo,hip-odoo/odoo,bplancher/odoo,optima-ict/odoo,ygol/odoo,storm-computers/odoo,laslabs/odoo,Elico-Corp/odoo_OCB,laslabs/odoo,dfang/odoo,syci/OCB,dfang/odoo,microcom/odoo,dfang/odoo,Elico-Corp/odoo_OCB,syci/OCB,microcom/odoo,ygol/odoo,storm-computers/odoo,ygol/odoo,laslabs/odoo,dfang/odoo,ygol/odoo,storm-computers/odoo,dfang/odoo,stephen144/odoo,storm-computers/odoo,optima-ict/odoo,stephen144/odoo,microcom/odoo,hip-odoo/odoo,laslabs/odoo,Elico-Corp/odoo_OCB,laslabs/odoo,Elico-Corp/odoo_OCB,ygol/odoo,optima-ict/odoo,bplancher/odoo,storm-computers/odoo,sysadminmatmoz/OCB,stephen144/odoo,microcom/odoo,hip-odoo/odoo,optima-ict/odoo,optima-ict/odoo,sysadminmatmoz/OCB,syci/OCB,bplancher/odoo,Elico-Corp/odoo_OCB,optima-ict/odoo,storm-computers/odoo,hip-odoo/odoo,hip-odoo/odoo,sysadminmatmoz/OCB,laslabs/odoo,ygol/odoo,sysadminmatmoz/OCB,ygol/odoo,syci/OCB,hip-odoo/odoo,stephen144/odoo,bplancher/odoo