Breaking changes in HCHttp programmatic config in 1.5

[rfoltyns]

HCHttp needs a major cleanup.

WIth incoming Service Discovery feature, most of the HCHttp.Builder class must be rewritten and HCHttp simplified. If will affect only programmatic config users and classes extending HCHttp - XML config will stay intact.

Let me know if you have any concerns.

[rfoltyns]

Here's a list of (un)planned changes:

• Initially, all protected HTTP config fields will be removed and replaced with HttpClientProvider (which was created by this class anyway)
• As mentioned above HttpClientProvider will become an independent class. Also, it will serve one HttpClient per instance instead of creating new HttpClient per call. It used to be package-private inner class. There no actual need for that. If you have package-private classes that extend it, migration should be quite easy.
• HCHttp.Builder will still allow "old-style" config of HTTP params, but all setters will now delegate to HttpClientProvider. Plus, they'll be deprecated and removed 1.6. Use withHttpClienProvider instead.
• In a long run, I'd like to have HCHttp class serving as "Generic HTTP batch handler" only and be completely stripped from Elasticsearch-specific details. It means that mappingType and itemSourceFactory will be moved to HCBatchOperations (where they actually belong). objectReader is next, but I haven't figured out how to deal with it yet.

Given all the above, it might actually make more sense to just introduce a HCHttpBatch class that would become new HCHttp Log4j2 plugin and deprecate the old class. This might keep all programmatic config afficionados happy. I'll make a decision once all the changes are in.

Let me know if you have any thoughts.

[rfoltyns]

Last major part of HCHttp refactoring is in 5016080. Generic batching logic was pulled up. The goal was to allow integration with other APIs.

• BatchOperations can be configured with HCHttp.Builder.withBatchOperations; HCHttp.mappingType and HCHttp.itemSouceFactory were removed as they belong to HCBatchOperations anyway. They're still available in HCHttp.Builder (until 1.6) if you need them in current extensions
• Batching logic and HTTP client call were pulled up to BatchingClientObjectFactory
• BatchingClientObjectFactory implements ClientObjectFactory
• Batch and Item interfaces were added
• IndexRequest doesn't implement Request interface any more (I can bring it back until 1.6 if requested). If necessary, it can be implemented with Operation interface. I'm not planning to support it - indexing documents one by one is out of scope of this library

XML configuration was not affected and this change is fully compatible with current configurations.

After a few more minor changes, it will be released in 1.5.0 in a few weeks.

Let me know if you have any concerns/suggestions/PRs.