Prefer unsynchronized StringBuilder

[gaul]

StringBuffer provides unneeded synchronization. Found via
error-prone.

[stleary]

What problem does this code solve?
Replaces an internally used inefficient StringBuffer class

Risks
Low

Changes to the API?
No

Will this require a new release?
No

Should the documentation be updated?
No

Does it break the unit tests?
No, and additional unit tests are not needed.

Was any code refactored in this commit?
No

Review status
Approved, starting 3 day comment window