Allow node operators to specify number of confirmations neded for L1 txns

[jacque006]

Problem

Currently, we consider all L1 transactions valid after 1 confirmations. In mainnet scenarios, this is rather naive and we need to wait for additional confirmations.

Relevant discussion: https://ethereum.stackexchange.com/questions/319/what-number-of-confirmations-is-considered-secure-in-ethereum

Solution

Allow node operators to specify how many confirmations they would like for L1 txns via a config field such as l1Confirmations. Default to 1 per current behavior.