Gateway sender has public setter methods that should not be exposed

Looking at the GatewaySender interface I noticed there are numerous public setter methods. Geode should not allow for the ability to directly change GatewaySender functionality without proper process.

This is largely to avoid the introduction of side effects into the system. A prime example of this is, the ability to call `setGroupTransactionEvents`, which from what I understand should NEVER be allowed to be changed in just 1 server instead of cluster-wide. This by writing a function and changing the setting on only 1 server can run the risk of the whole system behaving incorrectly causing failures which would be close to impossible to track down.