util: Add nodiscard to all {Decode,Parse}[...](...) functions returning bool

Summary:

• miner: Default to DEFAULT_BLOCK_MIN_TX_FEE if unable to parse -blockmintxfee
• tests: Check return value of ParseParameters(...)
• tests: Explicitly ignore the return value of DecodeBase58(...)
• Add NODISCARD to all {Decode,Parse}[...](...) functions returning bool. Sort includes.

This is a backport of Core PR13815

Depends on D4500

Test Plan: Build and make sure there is no arnign due to discarded result.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4501