Remove memory allocations from merkledb iteration

[StephenButtolph]

Why this should be merged

Improves the performance of Next() by ~70%. After this change, the only memory allocations in the iterate benchmark are performed in NewIterator or in memdb.iterator#Key(). So, Next() doesn't perform any memory allocations anymore.

Before:

BenchmarkIteration/create_iterator-12         	    4704	    257583 ns/op	   43179 B/op	       4 allocs/op
BenchmarkIteration/iterate-12                 	    1023	   1142045 ns/op	  267401 B/op	    6004 allocs/op

After:

BenchmarkIteration/create_iterator-12         	    5005	    239408 ns/op	   43228 B/op	       4 allocs/op
BenchmarkIteration/iterate-12                 	    3685	    331174 ns/op	   51229 B/op	    1004 allocs/op

How this works

We don't need to allocate the full dbNode for iteration. This pulls the Value out of the node without performing memory allocations.

How this was tested

• Added a new benchmark.
• CI