wallet: respect policy-level transaction size limits when crafting regular and funding transactions

[Roasbeef]

Tracking this here instead of the btcwallet issue for more visibility. As is, we don't always ensure that a transaction either we construct manually (for funding) or within btcwallet itself is below the current widely used policy that constraints transaction sizes. Thankfully, we can simply start using the existing policy level function within btcd itself to enforce these limits: https://github.com/btcsuite/btcd/blob/master/mempool/policy.go#L271

When a user encounters this issue, they'll see this in the logs:

: unmatched backend error: -26: 64: tx-size

And then end up with a dangling transaction that'll keep being rebroadcast, when instead we should remove it from the tx store.

[Roasbeef]

Actually, reading the btcwallet code more closely here, I think we already properly handle this case. What we don't do however, is remove the "pending open" channel from the lnd's DB.