Return the RTT parameters in the block template

Summary:
The parameters are chosen so that pool can either:

• Directly read the real time target value from the rtt.nexttarget field;
• Or compute it locally by using the other rtt.* parameters.

Depends on D16729.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, roqqit

Reviewed By: roqqit

Subscribers: PiRK, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D16730