Add support for a WAL

[armon]

If we add support for a write-ahead-log, then external tools can be used to setup log streaming and build a sane high availability setup.

[jf]

I know this is asking to pull in more pieces, but how about offloading this to a separate commit log system (something like Kafka), and then implementing a consumer for that instead? If the end (and only) objective is to do HA, that is.

[armon]

@jf Totally. Many ways to architect around this. At my old job where we used bloomd we simply had a separate log that went to S3 that we could replay in the event of a failure, so it was never a high priority to support native HA.

[onesuper]

It will be a great enhancement if a mechanism like 'binlog' is implemented internally.