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:

• [x] 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. (#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)
• [x] number of transactions for prev day (#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)