plog-common, with netty-friendly murmurhash3

plog-common should include code that doesn't fit in the API
or distribution, but benefits from being shared between many components.

As getting out of ByteBufs for checksumming turns out expensive,
but Guava doesn't expose any API allowing us to checksum Netty buffers,
get started with a naive implementation (hopefully not too naive).
plog-client: write header every time

From the stress tester, turns out copying a reference isn't particularly efficient...