This change adds an additional IsWaitingForConfirmation method to the
txmgr.SendState, which returns true if it is aware of at least one mined
tx. This allows the txmgr to skip publishing new txs while we are
waiting for additional confirmation on a mined tx, which in turn reduces
the number of useless API calls we make to the backend provider. As an
additional benefit, this also reduces the likelihood of a false positive
sample from ShouldAbortImmediately since it will stop processing errors
while we have a mined tx.