change (misleading) names of Hadoop metrics

[costin]

Unfortunately the current naming WRITTEN is misleading and folks assume it represents the number of documents written in ES instead of written to the wire.
Hence, there's a confusion of what accepted vs written vs rejected means.
To clarify this, we should rename WRITTEN to SENT (as in sent over the wire, sent to ES) and, in-tandem, rename READ to RECEIVED.