bench: Use non-throwing ParseDouble(...) instead of throwing boost::lexical_cast<double>(...)

Summary:

• Non-Boost is better than Boost.
• Non-throwing is better than throwing.
• Explicit error handling is better than implicit error handling.
• ParseDouble(…) deserves to be used outside of its unit tests :-)

Backport of Bitcoin Core PR13383
https://github.com/bitcoin/bitcoin/pull/13383

Test Plan:

make check-all

Reviewers: Fabien, #bitcoin_abc, deadalnix, nakihito

Reviewed By: #bitcoin_abc, deadalnix, nakihito

Differential Revision: https://reviews.bitcoinabc.org/D4080