RDS: make rds_conn_drop() take reason argument

RDS: make rds_conn_drop() take reason argument

This removes the need of modifying the conn all over the place
and moves it inside rds_conn_drop(). Actually there is almost
no need to carry the 'c_drop_source' information as part of
rds_connection but since shutdown thread wants to log this
info for debug,  the field is left as is for now.

Orabug: 22347191

Tested-by: Michael Nowak <michael.nowak@oracle.com>
Tested-by: Rafael Alejandro Peralez <rafael.peralez@oracle.com>
Tested-by: Liwen Huang <liwen.huang@oracle.com>
Tested-by: Hong Liu <hong.x.liu@oracle.com>
Reviewed-by: Mukesh Kacker <mukesh.kacker@oracle.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>