Initial row level filtering.

Proof of concept to get row level filtering in place. It's **very**
messy, I was focused on the how not how pretty :)

FilteredRangeScanHandler handles the HTTP request and creates a (splunk)
FilteredRangeSliceCommand. This command is passed to the SplunkStorageProxy
which processes it just like a RangeSliceCommand. Messages about the
command are delivered to the FilteredRangeSliceVerbHandler which processes
the call and returns a result for the SplunkStoreProxy to return to the
http handler.

The FilteredRangeSliceVerbHandler contains a lot of junk that I copied from
other parts of the C* code. Comments in the file describe where they are
from. The SSTableFilteredScanner does the actual low level filtering of rows,
for now it only includes rows that have an even timestamp.

This code relies on a fork of cassandra located at
https://github.com/amorton/cassandra/tree/splunk

To use the fork I made these changes to the intravert pom.xml to
point to my JAR and pull in the cassandra dependencies:

    <dependency>
        <groupId>com.splunk.cassandra</groupId>
        <artifactId>apache-cassandra</artifactId>
        <version>1.2.5-SNAPSHOT</version>
        <scope>system</scope>
        <systemPath>/Users/aaron/code/apache/cassandra/build/apache-cassandra-1.2.5-SNAPSHOT.jar</systemPath>
    </dependency>
    <dependency>
        <groupId>com.splunk.cassandra</groupId>
        <artifactId>apache-cassandra-thrift</artifactId>
        <version>1.2.5-SNAPSHOT</version>
        <scope>system</scope>
        <systemPath>/Users/aaron/code/apache/cassandra/build/apache-cassandra-thrift-1.2.5-SNAPSHOT.jar</systemPath>
    </dependency>
	<dependency>
		<groupId>org.apache.cassandra</groupId>
		<artifactId>cassandra-all</artifactId>
		<version>1.2.3</version>
        <type>pom</type>
	</dependency>