Refactor proxyservers out of ProxyUsers

HADOOP-10498 added proxyservers feature in ProxyUsers. It is beneficial to treat this as a separate feature since

1> The ProxyUsers is per proxyuser where as proxyservers is per cluster. The cardinality is different.

2> The ProxyUsers.authorize() and ProxyUsers.isproxyUser() are synchronized and hence share the same lock and impacts performance.

Since these are two separate features, it will be an improvement to keep them separate. It also enables one to fine-tune each feature independently.