Make ManagedLedger and ManagedCursor suit the custom plugin.

[horizonzy]

My repository PR link: horizonzy#19

Motivation

Now, we force cast class at the out side of module managed-ledger, if some users want to use the custom ManagedLedger, it will throw ClassCastException.
So we should only use the interface level out side of module managed-ledger.

Modifications

Extract some methods to the ManagedLedger interface when we need.

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• Dependencies (add or upgrade a dependency)
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