proto+lnrpc: update gRPC-REST gateway to v1.8.6

[guggero]

Fixes #3168

When using the REST gateway, fields that have the type bytes in rpc.proto cannot be used in query or path parameters (see #552 which lead to workarounds like #592).

The root cause of this is that the version of grpc-gateway we use doesn't support mapping to []byte yet.
Trying to use bytes parameters results in {"error":"unsupported field type uint8","code":3}.
This has been fixed in grpc-ecosystem/grpc-gateway#489.

Unfortunately, we can't use the newest version of grpc-gateway (which would be 1.11.3 at the moment) because that has a bug that messes up our rpc.swagger.json (grpc-ecosystem/grpc-gateway#746).

After some trial and error, I found that version 1.8.6 is the newest version that works for our use case and produces the smallest diff in our generated code.

[wpaulino]

Why is this required? This version seems to support go modules.

[guggero]

It's not required. That was left over from a trial with an older version. Fixed.

[joostjager]

it would be nice if we can also mark all the string fields as deprecated now and remove them in 0.10

[guggero]

@wpaulino no, he meant the fields introduced in #592 that worked around the bug that this PR fixes.

[cfromknecht]

nice, this actually reduces the size of our gateway code! LGTM 🔥

[halseth]

Tested and LGTM 👍

[joostjager]

it would be nice if we can also mark all the string fields as deprecated now and remove them in 0.10

Do we want to do this? I think it would be nice to be able to remove the string fields at some point

[halseth]

it would be nice if we can also mark all the string fields as deprecated now and remove them in 0.10

Do we want to do this? I think it would be nice to be able to remove the string fields at some point

Yep, can be done in separate PR though.

[joostjager]

I am getting a new error:

joost@joost-desktop $ make rpc
 Compiling protos.
cd ./lnrpc; ./gen_protos.sh
Generating root gRPC server protos
F1102 13:10:55.891383    8655 main.go:73] Cannot set flag paths=source_relative
--grpc-gateway_out: protoc-gen-grpc-gateway: Plugin failed with status code 255.

[joostjager]

Nvm, needed to update dep