Optimize sectors renew

[zlhwdsz]

Optimize subcommands check-expire and renew of lotus-miner sectors:

• Print timestamp instead of relative time in check-expire results with a customized epochTime function.

• Move util code to the end of the file.

• Decompose renew cmd for better readability.

• Reduce function calls in loops.

[zlhwdsz]

@ZenGround0 This PR would help a lot in terms of better readability and futrue maintenance.
diff may look like a mess so it's better to review the resulted file.