[kwsantiago]

When the backend's minimum relay fee is above 1 sat/vB, transactions paid at our floor feerate came out a few satoshis short and bitcoind rejected them. We round the fee down from weight, bitcoind rounds up from vsize, and FEERATE_FLOOR's slack only covers that gap at 1 sat/vB.

This adds feerate_relay_floor(), the lowest feerate that relays at a given backend minimum for any transaction weight from MINIMUM_TX_WEIGHT up, and uses it as the floor for transactions we pay for ourselves: estimates, the minimum feerate keyword, opening, mutual close, anchor fallbacks and onchain sweeps. Feerates we accept from peers still check against the raw backend minimum. At the default 1 sat/vB it returns FEERATE_FLOOR, so nothing changes there; above it, it adds about 0.75% plus 5 sat/kw.

Targets beyond 200 blocks used to return FEERATE_FLOOR regardless of the backend minimum. Delayed sweeps to our wallet always target 300 blocks out, so with a raised minimum they never relayed. They now get the relay floor too, and the onchain sweep loop stops at the relay floor rather than FEERATE_FLOOR.

Tests:

• run-feerate_relay_floor brute-forces the bound across backend minimums and transaction weights.
• test_penalty_htlc_min_feerate now also runs at raised minrelaytxfee values; 8 of its 10 variants failed before this change.
• test_onchain_delayed_sweep_min_feerate fails before this change ("min relay fee not met") and passes after.

The wallet weight under-count described in #67 did not reproduce: the funding estimate matches the transaction's actual weight, so there is no change for it.

Fixes #67

Changelog-None