Do not reject future blocks on speculative nodes

[heifner]

On speculative, non-block-producer, nodes do not reject blocks from the future. This allows a speculative node to have a clock skew without it interfering with maintaining synchronization. Before, this change a clock skew of 7 seconds would cause a speculative node to reject a block.

Note block producers still need to keep their clocks synchronized so they produce on time. Block producers will continue to reject blocks with timestamps >= 7 seconds in the future.

producer_plugin now calculates pending block time for speculative nodes to always be head.block_time().next() ignoring now.

Resolves #239

[ericpassmore]

Note:start
category: System Stability
component: Internal
summary: Enhance robustness of speculative block mode by allowing for future blocks. Future blocks commonly from clock-skew.
Note:end