Use String.concat in FailoverLoader for performance-critical concatenations of two Strings.

Use String.concat in FailoverLoader for performance-critical concatenations of two Strings.

This has performance improvements in fewer allocations in caliper tests.  Javac basically puts concatenations into a StringBuilder of size 16, so all sorts of random resizing goes on.  String.concat just allocates one new array sized appropriately.  Small thing, but it can make a difference. 
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