QDnsLookupRunnable: replace a volatile bool with an atomic int

QDnsLookupRunnable: replace a volatile bool with an atomic int

Since there is non-atomic data that is protected by 'triedResolve',
the (outer) read from triedResolve needs to have acquire, and the
store needs to have release semantics. The release implied by the
mutex unlock is not good enough because it only synchronises-with
the locking of the same mutex, which not all threads execute.

Change-Id: If46b3ea6ccfdd66ca41ce44d4f45bef2c2c30f72
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Shane Kearns <shane.kearns@accenture.com>