* trie: use pooling of iterator states in iterator
The node iterator burns through a lot of memory while iterating a trie, and a lot of
that can be avoided by using a fairly small pool (max 40 items).
name old time/op new time/op delta
Iterator-8 6.22ms ± 3% 5.40ms ± 6% -13.18% (p=0.008 n=5+5)
name old alloc/op new alloc/op delta
Iterator-8 2.36MB ± 0% 1.67MB ± 0% -29.23% (p=0.008 n=5+5)
name old allocs/op new allocs/op delta
Iterator-8 37.0k ± 0% 29.8k ± 0% ~ (p=0.079 n=4+5)
* ethdb/memorydb: avoid one copying of key
By making the transformation from []byte to string at an earlier point,
we save an allocation which otherwise happens later on.
name old time/op new time/op delta
BatchAllocs-8 412µs ± 6% 382µs ± 2% -7.18% (p=0.016 n=5+4)
name old alloc/op new alloc/op delta
BatchAllocs-8 480kB ± 0% 490kB ± 0% +1.93% (p=0.008 n=5+5)
name old allocs/op new allocs/op delta
BatchAllocs-8 3.03k ± 0% 2.03k ± 0% -32.98% (p=0.008 n=5+5)
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