[avalanche] Track the total size of finalized txs

Summary: This is expected to be our block size. It's a useful information on it's own, and gives option to implement a backpressure management method for tx polling.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17617