Add a notion of time to HCs.

[happi]

See https://www.erlang.org/doc/apps/erts/time_correction.html

• Integrate NTP (make sure the system runs it or directly in Erlang?)
• Remove any usage of erlang:now
• Decide on time to use for block timestamps
• Set rules for when a block is "in time"
• Calculate the timeslot for a block at a specific height.
• Tag incoming blocks (outside the block) with a "received time" tag.
• Introduce a network check to make sure your node, NTP, and other HC nodes in the network agree - give a "warning" or refuse to start if not.

[richcarl]

Sub-tasks:

• 4437