trie: improve the node iterator seek operation

[holiman]

This PR improves the efficiency of the nodeIterator seek operation.
Previously, the seek operation used the nextChild to iterate across the fullnode chldren, resolving each as it traversed towards the seek position.

Example:

marker: 01ee4cbd12051867841e785a715f770a635da089ff75b19eff5e6f719963e83f
trie.resolveHash resolve path 
INFO [03-09|14:20:27.177] Resuming state snapshot generation       root="349972…b86d77" at="01ee4c…63e83f" accounts=7 slots=0 storage=278.00B elapsed=1.875ms eta=263ms
trie.resolveHash resolve path 00
trie.resolveHash resolve path 0000
trie.resolveHash resolve path 0001
trie.resolveHash resolve path 000104
trie.resolveHash resolve path 000108
trie.resolveHash resolve path 00010e
trie.resolveHash resolve path 0002
trie.resolveHash resolve path 000206
trie.resolveHash resolve path 00020600
marker []: 0260c4f779bc30181efdd3639b6db85257cf2ee898aa2ffa6fd3ab675cf2fb0a

The marker it starts on is 00 01 0e .... . To find it, it resolves 00, 0000(!), 0001, 000104(!), 000108 (!) and 00010e. The ones marked with (!) are siblings that should not have to be resolved.
Since we resolve the fullnode parent (at 0001), it should be possible for the iterator to go directly to the 00010e child without resolving the sibling children.

So it does 9 lookups. After this PR, it does only 6: 00, 0001, 00010e, 0002, 000206, 00020600.:

marker: 01ee4cbd12051867841e785a715f770a635da089ff75b19eff5e6f719963e83f
trie.resolveHash resolve path 
INFO [03-09|16:27:20.719] Resuming state snapshot generation       root="349972…b86d77" at="01ee4c…63e83f" accounts=7 slots=0 storage=278.00B elapsed="704.754µs" eta=131ms
trie.resolveHash resolve path 00
trie.resolveHash resolve path 0001
trie.resolveHash resolve path 00010e
trie.resolveHash resolve path 0002
trie.resolveHash resolve path 000206
trie.resolveHash resolve path 00020600
marker []: 0260c4f779bc30181efdd3639b6db85257cf2ee898aa2ffa6fd3ab675cf2fb0a