Remove placeholders from connection when done

[G1gg1L3s]

This PR solves interference problem of placeholders. They are saved in the connection, but were never removed, only replaced. That means, they could affect other queries on the same connection.

Solve this, by removing the placeholders at the end of the query: when ReadyForQuery arrives.

I didn't write a new test for this, because 1) it already existed as a part of another test and 2) it's really hard to reproduce.

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)~