Fix multiple prepared stmt registraion

[G1gg1L3s]

This PR solves double registration of prepared statements. They are registered twice: after Parse and ParseComplete, but because of a race condition, the pending statement can be replaced by another. Then, this triggers registration of a nil value as a statement.

To solve the problem, this PR removes the second registration.

There is one issue though: prepared statements are never cleaned, even ones that contain errors. This requires a large redesigning of the whole packet processing protocol. So, for now, we have a small memory leakage in a connection. But on practice this should not be an issue.

Checklist

• [x] Change is covered by automated tests
• [x] The coding guidelines are followed
• [x] ~Public API has proper documentation in the Acra documentation site or has PR on documentation repository with new changes~
• [x] ~CHANGELOG.md is updated (in case of notable or breaking changes)~
• [x] CHANGELOG_DEV.md is updated
• [x] ~Benchmark results are attached (if applicable)~
• [x] ~Example projects and code samples are up-to-date (in case of API changes)~