Improve performance for checksum filter

Hi there,

I have some performance improvements for the checksum filter.  Here are the results from trying it on my machine:

Before:

    $ time yes 'a random sampling of a chunk of fake data' | head -20000 | ./bin/logstash agent -e 'input { stdin {} } filter { checksum { keys => "message" } } output { stdout {} }' >/dev/null
    yes 'a random sampling of a chunk of fake data'  0.00s user 0.00s system 0% cpu 1:38.38 total
    head -20000  0.00s user 0.00s system 0% cpu 1:38.38 total
    ./bin/logstash agent -e  > /dev/null  144.51s user 0.89s system 127% cpu 1:54.38 total

After:

    $ time yes 'a random sampling of a chunk of fake data' | head -20000 | ./bin/logstash agent -e 'input { stdin {} } filter { checksum { keys => "message" } } output { stdout {} }' >/dev/null
    yes 'a random sampling of a chunk of fake data'  0.00s user 0.00s system 0% cpu 17.883 total
    head -20000  0.00s user 0.00s system 0% cpu 17.882 total
    ./bin/logstash agent -e  > /dev/null  44.35s user 1.51s system 250% cpu 18.288 total