Code quality

[slowriot]

Description

This PR aims to make miscellaneous code quality improvements through the codebase, in conjunction with more substantive concrete fixes in PRs #53 and #54. Specifically:

• Disable the DEBUG_LOCKORDER define being set for debug build mode - the two should be unrelated, and the code gated by DEBUG_LOCKORDER is broken in any case
• Disable warnings on fallthrough, and address all instances where expected fallthrough occurs with the appropriate C++17 attribute - this improves readability and allows catching of future unintended fallthrough bugs
• Avoid implicit conversions that may alter a value, or make them explicit where that behaviour is desired. Add a placeholder warning to catch more such instances
• Resolve cases of unnecessary promotions (i.e. double promotion in calculating time interval) and add a warning to catch these in future - performance enhancement
• Remove unused includes, tidying the code and improving build time slightly
• Tidy and remove unneeded braces for switch cases that don't require them, improving readability
• Add compiler flags for additional safety and performance warnings

See commit messages for details of each change.

Note, the content of this PR was originally targeted against master, but the changes have also been added to a separate PR against the janus release branch shortly, to ensure consistency:

• Port multiple master-targeted fixes to janus release branch #56

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