DupSort of hash state

[AskAlexSharov]

Main idea:

CurrentStateBucket layout which utilises DupSort feature of LMDB (store multiple values inside 1 key).
LMDB stores multiple values of 1 key in B+ tree (as keys which have no values).
It allows do Seek by values and other features of LMDB.
You can think about it as Sub-Database which doesn't store any values.

-------------------------------------------------------------
       key              |            value
-------------------------------------------------------------
[acc_hash]              | [acc_value]
[acc_hash]+[inc]        | [storage1_hash]+[storage1_value]
                        | [storage2_hash]+[storage2_value] // this value has no own key. it's 2nd value of [acc_hash]+[inc] key.
                        | [storage3_hash]+[storage3_value]
                        | ...
[acc_hash]+[old_inc]    | [storage1_hash]+[storage1_value]
                        | ...
[acc2_hash]             | [acc2_value]
                        ...

On 10.5M block CurrentState bucket of this layout using 8Gb (40Gb in current layout)

Notice:

• This PR touching only hashed state, not plain state - to minimize risk. But env variable "DUPSORT_PLAIN" still works.
• Implementation is fully done inside kv_lmdb.go but in future I plan change KV interface closer to LMDB interface and move logic to Trie Loader. (feel free to push back and force me move logic to loader now)