Support for filters like SingleColumnValueFilter

[IceMan81]

Wondering if some of these filters are not implemented because people haven't found the need for it? I find that filters like SingleColumnValueFilter, SkipFilter can be useful.

Also any reason why some of the abstract methods on ScanFilter are package private?

[terensu]

`package org.hbase.async;

import org.hbase.async.generated.FilterPB;
import org.hbase.async.generated.HBasePB;
import org.jboss.netty.buffer.ChannelBuffer;

public class SingleColumnValueFilter extends ScanFilter {
private static final byte[] NAME = Bytes.UTF8("org.apache.hadoop.hbase.filter.SingleColumnValueFilter");

protected byte[] columnFamily;
protected byte[] columnQualifier;
protected CompareFilter.CompareOp compareOp;
protected BinaryComparator comparator;

public SingleColumnValueFilter(
        byte[] columnFamily, byte[] columnQualifier, CompareFilter.CompareOp compareOp, byte[] value) {
    this.columnFamily = columnFamily;
    this.columnQualifier = columnQualifier;
    this.compareOp = compareOp;
    this.comparator = new BinaryComparator(value);
}

@Override byte[] name() {
    return NAME;
}

@Override byte[] serialize() {
    return FilterPB.SingleColumnValueFilter.newBuilder().setColumnFamily(Bytes.wrap(columnFamily))
            .setColumnQualifier(Bytes.wrap(columnQualifier)).setComparator(comparator.toProtobuf())
            .setCompareOp(HBasePB.CompareType.valueOf(compareOp.name()))
            .setLatestVersionOnly(true)
            .setFilterIfMissing(true)
            .build().toByteArray();
}

@Override void serializeOld(ChannelBuffer buf) {
    //Nothing after hbase 0.94
}

@Override int predictSerializedSize() {
    return 0;//Nothing after hbase 0.94
}

}
`