Stop including all invoice r_tags in legacy trampoline onion

This change modifies create_trampoline_onion to only include as many
available r_tags as there is space left in the trampoline onion payload.

Previously we tried to include all passed invoice r_tags of legacy
trampoline payments into the payload which caused an user facing
exception and payment failure as the onion can only store a max of 400
bytes.
A single, single hop r_tag is around 52 bytes and the payload
without r_tags is already at ~280 bytes. So usually there is enough
space for 2 r_tags.
The implementation shuffles the r_tags on each call
so the payment will try different route hints on the attempts (fee level
increase or user retry).

I have logged the following byte sizes of the trampoline onion with a 2
trampoline onion hop and changing amounts of r_tags:

3 rtags:
payload size [0]: 113 (hop size: 81)
payload size [1]: 440 (hop size: 295) ( 52 bytes/rtag )
payload size [2]: 550 (hop size: 78)

2 rtags:
payload size [0]: 113 (hop size: 81)
payload size [1]: 386 (hop size: 241) ( 52 bytes/rtag )
payload size [2]: 496 (hop size: 78)

1 rtag:
payload size [0]: 113 (hop size: 81)
payload size [1]: 334 (hop size: 189) ( 52 bytes/rtag )
payload size [2]: 444 (hop size: 78)

0 rtags:
payload size [0]: 113 (hop size: 81)
payload size [1]: 282 (hop size: 137)
payload size [2]: 392 (hop size: 78)

As can be seen in the data, using 2 trampoline hops there is not enough
space for even a single r_tag which is why this option is being removed
too.

electrum/lnrouter.py

@@ -115,7 +115,7 @@ class RouteEdge(PathEdge):
 
 @attr.s
 class TrampolineEdge(RouteEdge):
-    invoice_routing_info = attr.ib(type=bytes, default=None)
+    invoice_routing_info = attr.ib(type=Sequence[bytes], default=None)
     invoice_features = attr.ib(type=int, default=None)
     # this is re-defined from parent just to specify a default value:
     short_channel_id = attr.ib(default=ShortChannelID(8), repr=lambda val: str(val))