Manage and share gateways of taskmanagers between jobs in session cluster

Flink session cluster launches a certain number of taskmanagers and they won't exit during the cluster running. When a job is submitted to the cluster, taskmanagers will offer slots to `JobMaster` with location info. Then `JobMaster` will get `InetAddress` from the location and connect the taskmanager. This cause some performance loss and it isn't necessary in session cluster.

It's better to manager the gateways of taskmanagers in JM and share them between jobs. When a taskmanager's gateway is exist and valid, the `JobMaster` can use it directly instead of creating a new one.