[CLI Feature] Estimated countdown to next block

It would be useful to quickly glance at the client status and known when the next block is likely to occur.

Implement a countdown to next block that shows the estimated number of seconds until the next block. This should be based on the consensus parameters and current state. If the countdown reaches zero, then it should remain there until a block is solved and the countdown starts again.

coda client status

Coda Daemon Status 

Global Number of Accounts:                     18
The Total Number of Blocks in the Blockchain:  1
Local Uptime:                                  36s
Estimated time to next block:                  19s

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