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
 * Embed sync.Mutex in the Proxier and serviceInfo types
 * 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 behavior have been introduced.

This is going to cause this message to get really spammy.  Let's make sure we only log when we're really stopping the service (move the log into stopProxyInternal?)