Fix double query execution

[lippserd]

Our monitoring list views call SimpleQuery::hasResult() first in
order to determine whether there are results to display. This calls
fetchRow() which executes the underlying query the first time. If there
are resulsts, the query is iterated which executes the query again.
With this patch, SimpleQuery::hasResult() makes use of the inner
iterator instead of calling fetchRow(). The query is now executed only
once.

Without patch

Description	Off (ms)	Dur (ms)	Mem (diff)	Mem (total)
Bootstrap, autoloader registered	0.003	0.003	682.06 KiB	682.06 KiB
Action::postDispatch()	260.597	260.594	7.01 MiB	7.68 MiB
Counting all results started	263.438	2.841	51.38 KiB	7.73 MiB
Counting all results finished	389.013	125.575	168.01 KiB	7.89 MiB
Fetching one row started	474.784	85.771	1535.91 KiB	9.39 MiB
Fetching one row finished	696.487	221.703	3.91 KiB	9.40 MiB
Query result iteration started	882.543	186.056	63.67 KiB	9.46 MiB
Query result iteration finished	911.221	28.678	391.45 KiB	9.84 MiB
Fetching one row started	912.754	1.533	34.02 KiB	9.87 MiB
Fetching one row finished	1003.547	90.793	2.43 KiB	9.88 MiB
Response ready	1100.532	96.985	292.98 KiB	10.16 MiB

With patch

Description	Off (ms)	Dur (ms)	Mem (diff)	Mem (total)
Bootstrap, autoloader registered	0.004	0.004	682.06 KiB	682.06 KiB
Action::postDispatch()	321.780	321.776	7.01 MiB	7.68 MiB
Counting all results started	325.437	3.657	51.38 KiB	7.73 MiB
Counting all results finished	471.803	146.366	168.01 KiB	7.89 MiB
Query result iteration started	789.899	318.096	1602.69 KiB	9.46 MiB
Query result iteration started	790.295	0.396	-8.55 KiB	9.45 MiB
Query result iteration finished	818.666	28.371	400.41 KiB	9.84 MiB
Fetching one row started	819.842	1.176	34.02 KiB	9.87 MiB
Fetching one row finished	916.380	96.538	2.12 KiB	9.88 MiB
Response ready	985.698	69.318	293.29 KiB	10.16 MiB