stats: prepare counters for main page

[sevenzing]

need to show info from stats

• on homepage:
  

• on /transactions page
  

• on /verified-contracts page
  

for now this UI takes this information from elixir backend.

main page:

• fix Average block time counter. for now it calculates average amount across all blocks. blockscout implements custom logic here: https://github.com/blockscout/blockscout/blob/e16236243195deb06d6d4b9e0bb67c534b2f04ad/apps/explorer/lib/explorer/counters/average_block_time.ex#L75-L81
  need to write the same thing. (feat(stats): The Migration of The Backend charts (Part 1: average_block_time) #1117)
• total blocks - to update almost in real time (as currently in blockscout)
• total transactions - to update almost in real time (as currently in blockscout)
• total addresses - to update almost in real time (as currently in blockscout)
• number of transactions for prev day (feat(stats): Back3nd chart5 Migration (Part 2: yesterday's transactions) #1120)
• daily transactions for 30 days chart

transactions page:

• transactions for 24h
• pending transactions
• transaction fees for 24h
• avg transaction fee 24h

verified contracts page:

• more frequent total contracts update
• new contracts for 24h
• more frequent total verified contracts update
• new verified contract for 24h

important: stats should calculate counters even if blockscout dint finish indexing yet

[bragov4ik]

note: added daily txns chart (pic). we figured out how to add it while letting it update immediately (w/o waiting for indexing)