Allow POST, PUT, PATCH and DELETE to be atomic on demand in Batch Requests

[AleksandrRogov]

Right now any POST, PUT, PATCH and DELETE will automatically be included into a Changeset which may not be useful in all situations.

Here are the tasks related to the issue:

• [x] Need to come up with logic that would allow developer to start a Changeset on demand, including having multiple separate changesets in a single batch operation. Need to think how it is going to look like and how to make it simple to understand and implement.
• [x] Implement the logic in the library.
• [x] Add/change unit tests related to batch requests.
• [x] Update Documentation

The new logic must not break existing logic of converting requests into a changeset by default. Mainly because it's going to be a very significant breaking change.

[AleksandrRogov]

The idea is to add an additional property inChangeSet to RequestBase interface that will default to true. It has to be set to false by the developer to execute an operation individually inside a batch request.

There are other ideas that I have but I doubt that I am going to include those in a release of v2:

• add nonAtomic property to startBatch. By default it will be false. If set to true - all operations in a batch request will be executed individually, unless inChangeSet has been set to true by the developer for that operation. Basically, nonAtomic = true reverses the default value for inChangeSet property to false when it's not set.
• add functions startChangeSet and endChangeSet which will have an effect only if nonAtomic in startBatch has been set to true. The idea here is to have this function for future major versions of the library and deprecate the current default logic where all operations (except for GET) are automatically included in the change sets (this logic was introduced in v1.5.0 when batch operations were added and I currently consider this logic to be incorrect). The correct way is to have individual operations in a batch request by default and if needed they can be included in the change sets by calling startChangeSet function or setting inChangeSet to true. Consequently, we should deprecate nonAtomic property in a Batch Request and remove with the next major version.

This ideas are up to debate. If anyone has any thoughts about it - let me know.