1190 bugfix blocksmith selection

[andy-shi88]

Description

inconsistent uses of previous block timestamp in determining next block's smithing time, caused the node won't continue to smith after stopped (even without reset)

Breakdown

• every calculation use estimated persist time
• cumulative difficulty calculated from skipped blocksmith instead of smithing round (which can cause 2 different quality block having same cumm-diff)
• calculate cummulative difficulty after block is sure to be persisted, so we don't calculate for blockpool block

Fix #1190

[iltoga]

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