Replace Pair with record in TableNameUtil.qualify

[keith-turner]

Looked for a place where using Pair would be more beneficial based on what I learned in #5906 and found the following.

accumulo/core/src/main/java/org/apache/accumulo/core/util/tables/TableNameUtil.java

Line 43 in 043bcba

public static Pair<String,String> qualify(String tableName) {

This method is used frequently and the code and has a lot of calls to getFirst and getSecond, where the namespace is the first and the tablename is second. This seems like a place where a record would be really beneficial.