Dex swap will kill the account

[zjb0807]

In a swap transaction, in comes from the sender and out goes to the sender, so the sender should keep alive

Case 1:
When a user has ACA tokens and has bound their EVM address, swapping all ACA for DOT will result in the user being automatically unbound from the EVM address, and the nonce will be reset.
Solution: The FE should keep the ED when swapping all ACA

Case 2:
When a user holds ACA and 0.011 DOT (0.01 ED), and uses all ACA to exchange for ERC20 tokens, the following scenarios occur:

• ACA is used to pay the transaction handling fee
• DOT is used to pay the EVM storage fee
• If the total funds do not meet the existential deposit (ED) requirements:
  • The account will be killed
  • The EVM address will be unbound
  • The ERC20 tokens will be transferred to a default address
  • The storage fee will be reserved on the sender
    https://acala.subscan.io/event?extrinsic=7439203-2

[xlc]

One solution is when account balances dropped to ED, put it to a queue instead of kill the account immediately. Then at end of the block (maybe on_idle), kill the account if the balances is still below ED