Support new Solr 4.8 nested documents

Problem/Motivation

There is a missing field and a missing cache which does not allow indexing nested documents.
Actually, without the field "_root_" we can still index, but when trying to update the index new documents will be created instead of just updating the existing ones.
The nested documents is a new feature introduced at Solr 4.8: http://heliosearch.org/solr-4-8-features/
An example data (in XML) is:

<add>
<doc>
  <field name="id">ISBN1_MASTER</field>
  <field name="label">Solr On Fire</field>
  <field name="bundle">parent</field>
  <field name="ss_ISBN">ISBN1</field>
        <doc>
                <field name="id">ISBN1_page1</field>
                <field name="bundle">child</field>
                <field name="ss_ISBN">ISBN1</field>
                <field name="ss_text">Welcome to Solr On Fire</field>
        </doc>
         <doc>
                <field name="id">ISBN1_page2</field>
                <field name="bundle">child</field>
                <field name="ss_ISBN">ISBN1</field>
                <field name="ss_text">Setting up Solr</field>
        </doc>
</doc>
<doc>
  <field name="id">ISBN2_MASTER</field>
  <field name="label">Mastering Solr</field>
  <field name="bundle">parent</field>
  <field name="ss_ISBN">ISBN2</field>
        <doc>
                <field name="id">ISBN2_page1</field>
                <field name="bundle">child</field>
                <field name="ss_ISBN">ISBN2</field>
                <field name="ss_text">Welcome to Mastering Solr</field>
        </doc>
         <doc>
                <field name="id">ISBN2_page2</field>
                <field name="bundle">child</field>
                <field name="ss_ISBN">ISBN2</field>
                <field name="ss_text">Setting up Solr Masterfully</field>
        </doc>
</doc>
</add>

Proposed resolution

Add the field "_root_" and add cache for nested documents.

Remaining tasks

patch to be submitted

User interface changes

none

API changes

none