proxy: cleanup and minor refactoring

This change includes minor refactoring and cleanup of the proxy
package including the following items:

 * Rename source files with misspelling of round robin
 * Remove unnecessary and redundant comments
 * Update comments for clarity
 * Add locking when updating the round-robin index
 * Improve method receiver names
 * Rename the LoadBalance method to NextEndpoint to add clarity

No changes in behaviour have been introduced.
Merge pull request #754 from kelseyhightower/proxy-cleanup

proxy: cleanup and minor refactoring