Implement feedback from CodeReview.SE

[dimo414]

Original report by Michael Diamond (Bitbucket: dimo414).

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See https://codereview.stackexchange.com/q/170958/14237

• Multi-tiered cache (Issue #1)
• Consider disown-ing background tasks - not sure if this is necessary or beneficial
• Avoid cat in favor of using Bash to read (while...do; ...; done <$file)
• Consider avoiding a mktemp'ed directory, possibly encoding $USER and/or $$ in the path
• Consider sqlite or some other backing store (likely not in V1)
• Avoid find where feasible (already in progress in 76e730b9)
• Consider alternative to current heredoc syntax, such as using 'single-quotes' and breaking out as needed ('$foo'"$bar"'$baz' -> $foo...$baz)
• No need for in "$@" in for loops

[dimo414]

Original comment by Michael Diamond (Bitbucket: dimo414).

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• disown-ing tasks doesn't appear necessary, advice is pretty consistent that (foo &) does what we need (1, 2, 3)
• Stopped using cat and subshells to read files in d056eed2

[dimo414]

Original comment by Michael Diamond (Bitbucket: dimo414).

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Calling this good, further tasks in #5.