Bug: balance check in router fails when providers of account changes during the trade

[dmoka]

Issue:

• in router we still do the balance check, but only check if the user spent at least the amount we calculate for the trade
• we use reducible_balance with mode Preservation.Expendable which behaves differently when the system.account.providers of the user account changes
• during this trade, the number of providers changes, increased by one, leading to a different behaviour for the balance check, namely an off-by-ED error

Fix would be to always use reducible_balance with Preservation.Preserve so the ED is always deducted from balances, resulting in consistent behaviour.

First we need to fix a bug in orml package: open-web3-stack/open-runtime-module-library#986

Cases on prod:

• FIXED: https://hydradx.subscan.io/extrinsic/4836337-2
• FIXED: https://hydradx.subscan.io/extrinsic/0x015c2692a4fc4a7e718e80b599fa87ca5ff9a50a734208e49932b380d4ad4967
• FIXED: https://hydradx.subscan.io/extrinsic/0x41db07453b8c6764a07953743327b7ef9533eff29d31eb76c76c038e860a3cca
• FIXED: https://hydradx.subscan.io/block/4898096?tab=event&&event=4898096-4

[dmoka]

Opened a PR for the fix in ORML: open-web3-stack/open-runtime-module-library#987

[dmoka]

It root cause is coming from other part too:

What happens is that the account of insufficient asset is killed, so the ED (1 HDX) is transferred back to the account. So our ensure_that_user_received_asset_out_at_most balance check fails, as the user have more balance with an ED.

Two solutions I can see:

• check if the insufficient asset account is killed, so we can incorporate ED in the balance check
• we discard the post balance checks. This might look easier first, but remember we added it back to prevent any issues like we had with stableswap a few weeks ago

[dmoka]

Related: #810