Reduce the number of charset lookups performed in BinaryValue under t…

[srgg]

Description

Getting rid of looking up UTF charset each time when
BinaryValue.createFromUtf8() and BinaryValue. toStringUtf8() is called

Motivation and Context

To speed-up corresponding operations

How Has This Been Tested?

It is expected that this change will be covered by existing tests

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)

Inapplicable

Checklist:

• My code follows the code style of this project.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• Javadoc comments for any new public classes, interfaces, etc.
• A basho_docs PR for new or changed features (basho/basho_docs#???)

Pull requests that are small and limited in scope are most welcome.