Avoid trap asset amount below a threshold

[xlc]

By default XCM pallet should not unconditionally trap assets. It should only trap assets with known value greater than some threshold to avoid dust storage issue. The storage will become economically inaccessible if the tx fee to claim trapped asset is greater than the assets to be claimed and those asset should just be dropped (or redirect to elsewhere).

Relates to paritytech/polkadot#4402

@KiChjang

[Polkadot-Forum]

This issue has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/asset-hub-brainstorming-session-call-notes-and-discussion-continuation/7260/5