This commit is a part of neo-project#2907: it implements conflict records storage scheme discribed
in neo-project#2907 (comment).

The short scheme description:

Do not store the list of conflicting signers in the Ledger's conflict record value.
Instead, put each conflicting signer in the conflict record key so that the
reculting key is: {Prefix_Transaction, <conflict hash>, <signer>}, and the value
is {<block index>}.

As an optimisation, for each conflict record store the dummy stub where the
key is {Prefix_Transaction, <conflict hash>} and the value is {<block index>}. This optimisation
allows to reduce DB read requests during newly-pooled transaction verification for
those transactions that do not have any on-chained conflicts.

Also, IsTraceableBlock check is added for on-chain conflicts verification. It is
needed to avoid situations when untraceable on-chained conflict affects the
newly-pooled transaction verification.

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

Malicious_OnChain_Conflict was constructed incorrectly (see the comment in the end of
the test) and was replaced by the proper TestMaliciousOnChainConflict test in
UT_Blockchain.cs way back ago in
neo-project@0c06c91.

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

This commit implements Conflicts attribute policy described in
neo-project#2907 (comment).

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

…conflicts

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

ConflictsFee logic was replaced by the generic attribute fee mechanism
implemented in neo-project#2916.

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

It's OK to keep them and save O(n) operations during OnPersist. The
storage taken by these malicious conflict records is properly paid
anyway. See the discussion under neo-project#2913 (comment).

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

…torage

`engine.Snapshot.Add` doesn't allow to rewrite storage entity if it's already exist.
Thus, we firstly need to remove it and afterwards to add the updated entity.

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

Use Snapshot.GetAndChange instead of subsequent calls to Delete and Add.
Ref. neo-project#2913 (comment).

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>