cope with one->many topology mappings

[rade]

We need this when mapping from container images to hosts - the same container image can reside on multiple hosts - and from pods to services - the same pod can be selected by multiple services. (Though the latter case currently isn't using joinResults.)

[rade]

I will squash the two commits. The intermediate commits shows the simple solution, but it causes too much of a performance degradation. Benchmarks from running

go test -tags 'netgo unsafe' -run=x -cpu=2 -benchmem -bench='Topology' --bench-report-path=/tmp/dev-on-dev-report.json

show (best results of three runs)...

master

BenchmarkTopologyList-2          	       5	 213608474 ns/op	83141875 B/op	  617248 allocs/op
BenchmarkTopologyHosts-2         	      20	 130394925 ns/op	55311573 B/op	  429426 allocs/op
BenchmarkTopologyControllers-2   	      20	 116441786 ns/op	45196386 B/op	  344697 allocs/op
BenchmarkTopologyPods-2          	      20	 104019790 ns/op	38841077 B/op	  289083 allocs/op
BenchmarkTopologyContainers-2    	      20	  75261315 ns/op	29496258 B/op	  217927 allocs/op
BenchmarkTopologyProcesses-2     	      30	  47699918 ns/op	14889463 B/op	   69993 allocs/op

intermediate

BenchmarkTopologyList-2          	       5	 218944680 ns/op	84870128 B/op	  647710 allocs/op
BenchmarkTopologyHosts-2         	      20	 137130296 ns/op	56615384 B/op	  452493 allocs/op
BenchmarkTopologyControllers-2   	      20	 121133337 ns/op	45773628 B/op	  355381 allocs/op
BenchmarkTopologyPods-2          	      20	 103948921 ns/op	39471932 B/op	  300246 allocs/op
BenchmarkTopologyContainers-2    	      20	  72434024 ns/op	29709738 B/op	  221733 allocs/op
BenchmarkTopologyProcesses-2     	      30	  48009831 ns/op	15090687 B/op	   73562 allocs/op

final

BenchmarkTopologyList-2          	       5	 216881152 ns/op	83195312 B/op	  617550 allocs/op
BenchmarkTopologyHosts-2         	      10	 131824903 ns/op	55332674 B/op	  429578 allocs/op
BenchmarkTopologyControllers-2   	      20	 118632501 ns/op	45110930 B/op	  343866 allocs/op
BenchmarkTopologyPods-2          	      20	 103424067 ns/op	38839952 B/op	  289083 allocs/op
BenchmarkTopologyContainers-2    	      20	  69692976 ns/op	29493521 B/op	  217925 allocs/op
BenchmarkTopologyProcesses-2     	      30	  48244260 ns/op	14888529 B/op	   69993 allocs/op