Reset the block template in the constructor

Summary:
This initalize the block template with the reserved coinbase size when constructed. There is no change in behavior because resetBlock() is already called in the callsites, but it makes the code easier to reason about.

Depends on D17791.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17792