Fix MySQL metadata when prepared statement caching is enabled

[wilkinsona]

Fixes gh-549

What does this PR do?

Fixes the MySQL metadata to avoid a failure when cachePrepStmts is true.

Code sections where the PR accesses files, network, docker or some external service

The newly added test uses Docker, but only in the same way as the existing tests.

Checklist before merging

• I have considered including reachability metadata directly in the library or the framework (see our contributing document)
• I am the original author of all content provided in the pull request, and I did not copy the content from any other source (see the tests section from our contributing document)
• For all tests where I am not the sole author, I have added a comment that proves I may publish them under the specified license (see the tests section from our contributing document)
• I have properly formatted metadata files (see our formatting guide)
• I have added thorough tests (see the tests section from our contributing document)
• I have filled all places where my pull request accesses files, network, docker, or any other external service (see the section above)