implement sync CoveredNetworksWalk with callback

[themiron]

It's useful for cases where walk needs to stop after Nth found network w/o traversing the whole network list.
Accodrding bench, it's even faster than current goroutine-based variant:

  Benchmark_CoveredNetworksAsync/IPTrie-12   2  829524748 ns/op  10000 batch_size
  Benchmark_CoveredNetworksSync/IPTrie-12   91   13892012 ns/op  10000 batch_size

With stop after the first found, it's obviously faster too:

  Benchmark_CoveredNetworksAsync/IPTrie-12  2        804893434 ns/op  10000 batch_size
  Benchmark_CoveredNetworksSync/IPTrie-12   2740110  443.1 ns/op      10000 batch_size