Consider adaptive required accumulated difficulty

[nkuba]

Currently, we determine the required accumulated difficulty by multiplying difficulty from transaction block by 6.

We need to provide a more reliable calculation.

Originated from: https://github.com/keep-network/tbtc-dapp/pull/8#discussion_r307543781

[mhluongo]

We're sticking with 6 confirmations for v1. We'll consider variable difficulty based on deposit velocity in v2