bpf/proxy: do not overwrite derived if unchanged
bpf/proxy: do not overwrite derived if unchanged

Do the full overwrite on startup sync as we want to have a good
consisten state to start from and it is sometimes complicated to say
what matches what as the BPF maps do not carry the full information.

before:

BenchmarkServiceUpdate/Services_10000_Endpoints_1_mockMaps_true-12                            92          12931154 ns/op         3704672 B/op         31 allocs/op
BenchmarkServiceUpdate/Services_10000_Endpoints_1_mockMaps_false-12                           99          13093264 ns/op         3705077 B/op         57 allocs/op
BenchmarkServiceUpdate/Services_10000_Endpoints_1_mockMaps_true_+_derived-12                  39          30952133 ns/op        10816597 B/op      70317 allocs/op
BenchmarkServiceUpdate/Services_10000_Endpoints_1_mockMaps_false_+_derived-12                 21          53796421 ns/op        12633100 B/op     150328 allocs/op
BenchmarkServiceUpdate/Services_10000_Endpoints_10_mockMaps_true-12                           63          21943512 ns/op         3705469 B/op         85 allocs/op
BenchmarkServiceUpdate/Services_10000_Endpoints_10_mockMaps_false-12                          63          22050243 ns/op         3708528 B/op        226 allocs/op
BenchmarkServiceUpdate/Services_10000_Endpoints_10_mockMaps_true_+_derived-12                 27          39987153 ns/op        10815805 B/op      70330 allocs/op
BenchmarkServiceUpdate/Services_10000_Endpoints_10_mockMaps_false_+_derived-12                22          60027131 ns/op        12634926 B/op     150525 allocs/op

after:

BenchmarkServiceUpdate/Services_10000_Endpoints_1_mockMaps_true-12               	      84	  12613745 ns/op	 3705096 B/op	      32 allocs/op
BenchmarkServiceUpdate/Services_10000_Endpoints_1_mockMaps_false-12              	      98	  12628490 ns/op	 3705673 B/op	      58 allocs/op
BenchmarkServiceUpdate/Services_10000_Endpoints_1_mockMaps_true_+_derived-12     	      46	  27247028 ns/op	10417729 B/op	   30336 allocs/op
BenchmarkServiceUpdate/Services_10000_Endpoints_1_mockMaps_false_+_derived-12    	      49	  29184740 ns/op	10417524 B/op	   30374 allocs/op
BenchmarkServiceUpdate/Services_10000_Endpoints_10_mockMaps_true-12              	      62	  21360122 ns/op	 3705309 B/op	      85 allocs/op
BenchmarkServiceUpdate/Services_10000_Endpoints_10_mockMaps_false-12             	      62	  22940588 ns/op	 3708641 B/op	     226 allocs/op
BenchmarkServiceUpdate/Services_10000_Endpoints_10_mockMaps_true_+_derived-12    	      36	  35517080 ns/op	10422811 B/op	   30395 allocs/op
BenchmarkServiceUpdate/Services_10000_Endpoints_10_mockMaps_false_+_derived-12   	      37	  37341294 ns/op	10421588 B/op	   30546 allocs/op

neiljerram/felix,neiljerram/felix,Metaswitch/calico,neiljerram/felix,neiljerram/felix,Metaswitch/calico