stats: utilize transaction_stats when available

[bragov4ik]

Apparently, transaction_stats has useful precomputed statistics for recent days. We might use it instead of heavy (30+ mins in eth mainnet) queries (if available).

Potential affected charts:

• txns_fee
• average_txn_fee
• average_gas_price

[bragov4ik]

Some details from DMs with Victor:

• This table is used & updated
• It only contains data for some recent days (depends on TXS_STATS_DAYS_TO_COMPILE_AT_INIT variable, although the env is probably not relevant)
• It updates once each day at 0:00 UTC

[bragov4ik]

From this info, it makes sense to update corresponding charts after this data becomes available.

Although it doesn't perfectly fit with existing model of update, it's possible to make a custom logic for such case; need to think about it
(i.e. it's not trivial to do properly)