Fix synchronous_mode/synchronous_mode_strict ignored when set to false

[drivebyer]

In my use case, I import the package from https://github.com/zalando/postgres-operator/tree/master/pkg/apis/acid.zalan.do/v1 into my Golang project. In my project, I use Golang to populate the PostgreSQL resource, and then use client-go to create the resource. However, when updating the PostgreSQL resource, like setting spec.patroni.synchronousMode to false, it gets ignored after marshaling to a string because of the omitempty tag. In my opinion, these fields should not have the omitempty tag.