[FEATURE] Introduce Empty Block Period

[purplesmoke05]

Is your feature request related to a problem? Please describe.

• [FEATURE] Introducing the Empty Block Period ibet-Network#126
• In the above PR, the Empty Block Period will be introduced.
• Accordingly, the default threshold value for block sync monitoring processor monitoring needs to be changed.

Describe the solution you'd like

• Change the default threshold value for block sync monitoring processor monitoring.
  • Change EXPECTED_BLOCKS_PER_SEC from a constant to an environment variable #686
  • Change the default value of EXPECTED_BLOCKS_PER_SEC from 1 sec to 10 sec