multi: add relative thaw height interpretation

This is useful when we wish to have a channel frozen for a specific
amount of blocks after its confirmation. This could also be done with an
absolute thaw height, but it does not suit cases where a strict block
delta needs to be enforced, as it's not possible to know for certain
when a channel will be included in the chain. To work around this, we
add a relative interpretation of the field, where if its value is below
500,000, then it's interpreted as a relative height. This approach
allows us to prevent further database modifications to account for a
relative thaw height.

lntest/itest/lnd_test.go

@@ -14501,11 +14501,6 @@ func testExternalFundingChanPoint(net *lntest.NetworkHarness, t *harnessTest) {
 		t.Fatalf("unable to gen pending chan ID: %v", err)
 	}
 
-	_, currentHeight, err := net.Miner.Node.GetBestBlock()
-	if err != nil {
-		t.Fatalf("unable to get current blockheight %v", err)
-	}
-
 	// Now that we have the pending channel ID, Dave (our responder) will
 	// register the intent to receive a new channel funding workflow using
 	// the pending channel ID.
@@ -14514,7 +14509,7 @@ func testExternalFundingChanPoint(net *lntest.NetworkHarness, t *harnessTest) {
 			FundingTxidBytes: txid[:],
 		},
 	}
-	thawHeight := uint32(currentHeight + 10)
+	thawHeight := uint32(10)
 	chanPointShim := &lnrpc.ChanPointShim{
 		Amt:       int64(chanSize),
 		ChanPoint: chanPoint,