This commit adds the following metrics to the DataPoster:

arb/dataposter/nonce/unconfirmed: current unconfirmed nonce
arb/dataposter/nonce/finalized: nonce as of finalized block
arb/dataposter/nonce/softconfirmed: soft confirmed nonce
      ie nonce as of latest block number - nonce-rbf-soft-confs
arb/dataposter/queue/length: number of unconfirmed batches in the queue
arb/dataposter/queue/weight: weight of unconfirmed batches in the queue

All are udpated every 10 seconds except softconfirmed, which is updated
is updated whenever there is a new batch posted or a tx is replaced.