Cross facet, faceting on multiple columns.

We often come across the scene of the multi-faceted cross, For example, the SQL statement, select count( * ) from table group by A,B.

Now we slightly modified for FacetComponent, this component to be able to support the multi-faceted cross.you can facet on multiple columns, and get the count result of multi-faceted cross.

Request parameters are as follows：
&start=0&rows=0&q=:&facet=true&facet.field=user_city&facet.field=user_province&facet.limit=10&facet.cross=true&facet.cross.sep=,

The original effect is as follows:
<result name="response" numFound="479140" start="0" sum="0.0" max="-Infinity" min="Infinity"/>
<lst name="facet_counts">
<lst name="facet_queries"/>
<lst name="facet_fields">
<lst name="user_city">
<int name="Beijing">16852</int>
<int name="ShangHai">16787</int>
<int name="Gunagzhou">12950</int>
<int name="Shenzhen">11667</int>
<int name="Hangzhou">9997</int>
<int name="Chongqing">7624</int>
<int name="Chengdu">7082</int>
<int name="Wuhan">6894</int>
<int name="Suzhou">6528</int>
<int name="Tianjin">5822</int>
</lst>
<lst name="user_province">
<int name="Gunagdong">48621</int>
<int name="Zhengjiang">34634</int>
<int name="Jiangsu">28748</int>
<int name="Shandong">20389</int>
<int name="Fujian">18508</int>
<int name="Beijing">16852</int>
<int name="Shanghai">16787</int>
<int name="Hubei">15227</int>
<int name="Sichuan">15112</int>
<int name="Hebei">13793</int>
</lst>
</lst>
<lst name="facet_numTerms"/>
<lst name="facet_dates"/>
<lst name="facet_ranges"/>
</lst>

The effect of the new features are as follows：

<result name="response" numFound="479140" start="0" sum="0.0" max="-Infinity" min="Infinity"/>
<lst name="facet_counts">
<lst name="facet_queries"/>
<lst name="facet_fields">
<lst name="user_city,user_province">
<int name="Beijing,Beijing">16852</int>
<int name="Shanghai,Shanghai">16787</int>
<int name="Guangzhou,Gunagdong">12950</int>
<int name="Shenzheng,Guangdong">11667</int>
<int name="Hangzhou,Zhejiang">9997</int>
<int name="Chongqing,Chongqing">7624</int>
<int name="Chengdu,Sichuan">7082</int>
<int name="Wuhan,Hubei">6894</int>
<int name="Suzhou,Jiangsu">6528</int>
</lst>
</lst>
<lst name="facet_numTerms"/>
<lst name="facet_dates"/>
<lst name="facet_ranges"/>
</lst>
</response>