resolved: add 'next_attempt_after' field to DnsTransaction

resolved: add 'next_attempt_after' field to DnsTransaction

For each transaction, record when the earliest point in time when the
query packet may hit the wire. This is the same time stamp for which
the timer is scheduled in retries, except for the initial query packets
which are delayed by a random jitter. In this case, we denote that the
packet may actually be sent at the nominal time, without the jitter.

Transactions that share the same timestamp will also have identical
values in this field. It is used to coalesce pending queries in a later
patch.