[Neo Store UT] Add leveldb thread UT and readme. · neo-project/neo@c01ab99

'NeoSystem.GetSnapshot()' is obsolete: 'This method is obsolete, use GetSnapshotCache instead.'

'NeoSystem.GetSnapshot()' is obsolete: 'This method is obsolete, use GetSnapshotCache instead.'

Converting null literal or possible null value to non-nullable type.

Possible null reference argument for parameter 'ret' in 'void MainService.OnBroadcastCommand(MessageCommand command, ISerializable ret)'.

Converting null literal or possible null value to non-nullable type.

Dereference of a possibly null reference.

'NeoSystem.GetSnapshot()' is obsolete: 'This method is obsolete, use GetSnapshotCache instead.'

'NeoSystem.GetSnapshot()' is obsolete: 'This method is obsolete, use GetSnapshotCache instead.'

'NeoSystem.GetSnapshot()' is obsolete: 'This method is obsolete, use GetSnapshotCache instead.'

'NeoSystem.GetSnapshot()' is obsolete: 'This method is obsolete, use GetSnapshotCache instead.'

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