Cache symmetric keys in same way as asymmetric

[Lagovas]

Found that our keystore cache only asymmetric keys in in-memory cache and ignore symmetric keys. And it increases the number of syscalls to read files and reduces performance. In this PR add caching symmetric keys too.

Checklist

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

[vixentael]

🧋 👍