✨ [ES] - Code refactorings: cleaning old code and improving readability

[Agnul97]

Main refactorings:

1. Safer multi-threading inizialitation of elasticSearchClient inside RestElasticsearchClientProvider
2. Removed no more useful code inside RestElasticsearchClientProvider (regarding a useless internalElasticsearchRestClient that was actually never used) and removed a not used logic to re-connect such client if missing
3. Renamed RestElasticsearchClient into RestElasticsearchClientWrapper
4. deleted no more needed abstraction AbstractElasticsearchClient (the only client now used is the REST one and probably will be this also in the future)

[codecov-commenter]

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Codecov Report

:x: Patch coverage is 0% with 35 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ch/client/rest/RestElasticsearchClientWrapper.java	0.00%	16 Missing :warning:
...h/client/rest/RestElasticsearchClientProvider.java	0.00%	11 Missing :warning:
.../elasticsearch/client/ElasticsearchRepository.java	0.00%	4 Missing :warning:
.../kapua/service/datastore/steps/DatastoreSteps.java	0.00%	4 Missing :warning:

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