Allow sending whole balance through XCM

[jak-pan]

Currently we don't allow the accounts to get under existential deposit. We should allow users to wipe their whole accounts by transferring all. The problem is specifically with higher value assets like DOT. Which needs to either leave 1DOT on relay or 0 otherwise users could lose fairly high value ~<10$ by sending wrong increment.

TLDR; allow sending whole balance to wipe accounts.
Make sure we don't wipe value from accounts. (Send all or as much as possible while doing so)