Clean up LMDB values accesses

Description

From the commit message:

The values, including the character strings, returned by LMDB are not zero-terminated. Therefore, one must specify the number of characters to use when constructing the std::string object. Otherwise, the code may or may not work depending on the memory content.

Related Issue

Fixes #105 (closed) without any pretension to be the final answer.

How Has This Been Tested?

Compiles, still able to launch a run.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.