Support nested child rows

[jtaylor-sfdc]

Unlike with standard relational databases, HBase allows you the flexibility of dynamically creating as many key values in a row as you'd like. Phoenix could leverage this by providing a way to model child rows inside of a parent row. The child row would be comprised of the set of key values whose column qualifier is prefixed with a known name and appended with the primary key of the child row. Phoenix could hide all this complexity, and allow querying over the nested children through joining to the parent row.