add new max channel size config option

- let users specify their MAXIMUM WUMBO with new config option which sets the maximum channel size lnd will accept
- current implementation is a simple check by the fundingManager rather than anything to do with the ChannelAcceptor
- Add test cases which verify that maximum channel limit is respected for wumbo/non-wumbo channels
- use --maxchansize 0 value to distinguish set/unset config. If user sets max value to 0 it will not do anything as 0 is currently used to indicate to the funding manager that the limit should not be enforced. This seems justifiable since --maxchansize=0 doesn't seem to make sense at first glance.
- add integration test case to ensure that config parsing and valiation is proper. I simplified the funding managers check electing to rely on config.go to correctly parse and set up either i) non wumbo default limit of 0.16 BTC OR ii) wumbo default soft limit of 10 BTC

Addresses: https://github.com/lightningnetwork/lnd/issues/4557

lnwallet/errors.go

@@ -142,6 +142,16 @@ func ErrChanTooSmall(chanSize, minChanSize btcutil.Amount) ReservationError {
 	}
 }
 
+// ErrChanTooLarge returns an error indicating that an incoming channel request
+// was too large. We'll reject any incoming channels if they're above our
+// configured value for the max channel size we'll accept.
+func ErrChanTooLarge(chanSize, maxChanSize btcutil.Amount) ReservationError {
+	return ReservationError{
+		fmt.Errorf("chan size of %v exceeds maximum chan size of %v",
+			chanSize, maxChanSize),
+	}
+}
+
 // ErrHtlcIndexAlreadyFailed is returned when the HTLC index has already been
 // failed, but has not been committed by our commitment state.
 type ErrHtlcIndexAlreadyFailed uint64