It is not possible to remove some routes

[Senyoret1]

Describe the bug
When trying to remove some routes the operation fails and the visor is terminated with an error.

Environment information:

• OS: Linux (Ubuntu 18.04.1)
• Platform: Linux 4.15.0-65-generic x86_64

Steps to Reproduce
Steps to reproduce the behavior:

1. Start some nodes using the make integration-run-generic command of the skywire-services repository (tested using the milestone2 branch of the skywire-mainnet repo and the mainnet-milestone2 branch of the skywire-services repo).
2. Create transports connecting the nodes by calling POST /api/visors/0348c941c5015a05c455ff238af2e57fb8f914c399aab604e9abb5b32b91a4c1fe/transports with {"remote_pk":"024ec47420176680816e0406250e7156465e4531f5b26057c9f6297bb0303558c7","transport_type":"dmsg","public":true} as content and then again with {"remote_pk":"031b80cd5773143a39d940dc0710b93dcccc262a85108018a7a95ab9af734f8055","transport_type":"dmsg","public":true} as content.
3. On the command window used to run make integration-run-generic run now curl --data {'"recipient":"'$PK_A'", "message":"Hello Joe!"}' -X POST $CHAT_C. This will send a msg to VisorA (it will appear on the console) and 2 routes will be added to each visor.
4. Call DELETE /api/visors/0348c941c5015a05c455ff238af2e57fb8f914c399aab604e9abb5b32b91a4c1fe/routes/1 API endpoint to remove the route with key 1 from the 0348c... visor (VisorB).

Actual behavior
When trying to delete the route from the 0348c... visor the API will respond "error": "unexpected EOF" and the command window for VisorB will show that the visor was stopped after an error.

The problem happens only with some routes. In fact, different routes have different behaviors. If during step 4 you call DELETE /api/visors/031b80cd5773143a39d940dc0710b93dcccc262a85108018a7a95ab9af734f8055/routes/1 for removing the route with key 1 from the 031b8... visor (VisorC), the route will be removed. If you call DELETE /api/visors/031b80cd5773143a39d940dc0710b93dcccc262a85108018a7a95ab9af734f8055/routes/2 for removing the route with key 2 from the 031b8... visor, not only the route, but all the routes on all the visors will be removed.

Expected behavior
It should be possible to remove any route without problems.

Additional context
The problem also appears if you remove the transports and then try to use the routes by running curl --data {'"recipient":"'$PK_A'", "message":"Hello Joe!"}' -X POST $CHAT_C again.

Found while testing #161 , not sure if related.

Possible implementation