[#164] Use polling instead '/monitor/heads' for bootstrapping node

Description

Problem: While node is bootstrapping, the latest head level on node tile updates very quick which leads to the UI hanging.

Solution: Poll /blocks/head endpoint every 5 seconds while node is syncing and then switch to listening /monitor/heads endpoint.

Related issue(s)

Relates #164 (closed)

Сhecklist

• I checked whether I should update
  • Root README.
  • Documents within docs.
• I checked whether my changes affect DB columns and tested DB migration using this doc.