Fix build on Rust 1.45.0 and up

[niklaslong]

This fixes the f64 to u64 conversion and associated bounds for the outgoing amount in the exchange rate calculation.

Included:

• updated deps (cargo update)
• set type length limit so that api crate would compile
• fixed f64 to u64 undefined behaviour as Rust 1.45.0 defines out of range float to int conversions using as as a saturating conversion.
• updates CI to use Node 12.18.4 (the JS ilp-settlement-xrp package which used the ripple-binary-codec lib is no longer compatible with Node 11x)
• CLI documentation for account update and account update-settings
• clippy fixes
• cargo audit fixes: see FIXME in CI config. All offending deps have been ignored (beyond the scope of this PR). net2 has been replace with socket2 in interledger crates, though is still present in tokio: https://github.com/tokio-rs/tokio/issues/2677

[niklaslong]

@ljedrz CI fails (no test is actually failing, might be because circleci is having some issues?) though it's green locally. I don't have the permissions to re-run the build on this.

In addition to the fixes discussed, I've also fixed a new 1.47 release related clippy lint.