Use system sodium in the crypto function by default: [ECR-3288]

[dmitry-timofeev]

Overview

That allows the users to benefit from the system libsodium
whenever it is available (it always is when the app is run,
but might not be in other usages).

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See: https://jira.bf.local/browse/ECR-3288

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 increased (+0.003%) to 85.675% when pulling 3eb4b3b on dmitry-timofeev:use-system-sodium-by-default-ECR-3288 into cdc4a17 on exonum:master.

[bullet-tooth]

I believe that com.exonum.binding.common.crypto.CryptoFunctions#ed25519 javadoc should be updated as well

[dmitry-timofeev]

javadoc should be updated as well

How exactly? The system library thing?

[bullet-tooth]

Something like:

  /**
   * Returns a ED25519 public-key signature system crypto function.
   * If system crypto library is not found then the bundled one is returned.
   */