[ISSUE #131] Support Snapshot for Dledger

[tsunghanjacktsai]

Test List

• [x] Unit testing, functional testing of each interface.
• [x] Test loading snapshot after node restart.
• [x] Test starting the DLedger under normal mode without a state machine.
• [x] Test if the number of reserved snapshots meets the specified requirements.
• [x] Test if previous snapshots can be loaded once the current one fails.

[RongtongJin]

Hi, @hzh0425 @mxsm @TheR1sing3un Plz help review this PR if you have time.

[TheR1sing3un]

Hi~ Please resolve the conflicts if you have time~