We gratefully acknowledge B. Hoare, F. Rudolph and C. Strickland-Constable for helpful discussions. FH and SD would like to thank the organisers of Corfu Summer Institutes 2018 where some results of this work were presented, and the COST network for supporting the visit of FH to this workshop. The work of DCT is supported by a Royal Society University Research Fellowship URF 150185, and in part by the STFC grant ST/P00055X/1. FH is partially supported by the Spanish Government Research Grant FPA2015-63667-. SD is supported by the FWO-Vlaanderen through aspirant fellowship and in part through the project FWO G020714N.