Consider maximum path length limit during path finding.

[tnull]

Currently, the maximum length of candidate paths is not considered during path finding, which could result in some paths being returned from get_route that are then immediately failing during construction of the onion. (Note: The only place the max. length is currently mentioned is this comment, which however refers to the pre-TLV maximum of 20.)

To solve this, we should try to approximate the TLV-onion length of candidate paths in get_route and pre-filter the results so that other viable paths are preferred.