Make CoreSchema a plain schema (not a proxy) [ECR-3865]

[dmitry-timofeev]

Overview

Make CoreSchema a plain schema — a factory of Exonum indexes.

The existing ITs in BlockchainIntegrationTest adequately cover
the operations now implemented in Java CoreSchema.

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See: https://jira.bf.local/browse/ECR-3865

Definition of Done

• There are no TODOs left in the code
• Change is covered by automated tests
• The coding guidelines are followed
• Public API has Javadoc
• Method preconditions are checked and documented in the Javadoc of the method
• Changelog is updated if needed (in case of notable or breaking changes)
• The continuous integration build passes

[coveralls]

Coverage decreased (-0.05%) to 86.818% when pulling 6bb5bdb on dmitry-timofeev:java-only-core-schema-ECR-3865 into 5287b2b on exonum:master.