Client

Adds support for compatibility header for Elasticsearch. If the environment variable 'ELASTIC_CLIENT_APIVERSIONING' is set to true or 1, the client will send the headers Accept and Content-Type with the following value: application/vnd.elasticsearch+json;compatible-with=7.
Favor POST method when only GET & POST method are available to prevent goroutine leak. #250
Filter master only nodes in discovery. #256

API

New APIs:
- FeaturesResetFeatures
- IngestGeoIPStats
New experimental APIs:
- ShutdownGetNode
- ShutdownPutNode
- ShutdownDeleteNode

New APIs:
- MLDeleteTrainedModelAlias
- MLPreviewDataFrameAnalytics
- MLPutTrainedModelAlias
APIs promoted to stable:
- TextStructureFindStructure
- MLDeleteDataFrameAnalytics
- MLDeleteTrainedModel
- MLEvaluateDataFrame
- MLExplainDataFrameAnalytics
- MLGetDataFrameAnalytics
- MLGetDataFrameAnalyticsStats
- MLGetTrainedModels
- MLGetTrainedModelsStats
- MLPutDataFrameAnalytics
- MLPutTrainedModel
- MLStartDataFrameAnalytics
- MLStopDataFrameAnalytics
- MLUpdateDataFrameAnalytics
New Beta APIs:
- SecurityCreateServiceToken,
- SecurityClearCachedServiceTokens
- SecurityDeleteServiceToken,
- SecurityGetServiceAccounts,
- SecurityGetServiceCredentials,
New experimental APIs:
- SearchableSnapshotsCacheStats,