memcoin proxy start is not idempotent

[augustebaum]

To reproduce:

1. In the project root, run start_test.sh.
2. In the pane not reserved for the running nodes, run all the commands in setup.sh until

memcoin --config /tmp/node1 proxy start --clientaddr 127.0.0.1:8081

3. Run this command now. Everything should work.
4. Run this command again. Node 1 panics with the following stack trace:

<datetime> PNC /path/to/go/pkg/mod/go.dedis.ch/dela@v0.0.0-20211018150429-1fdbe35cd189/mino/proxy/http/mod.go:108 >
failed to create conn '127.0.0.1:8081': listen tcp 127.0.0.1:8081: bind: address already in use role="http proxy"
panic: failed to create conn '127.0.0.1:8081': listen tcp 127.0.0.1:8081: bind: address already in use

goroutine 438 [running]:
github.com/rs/zerolog.(*Logger).Panic.func1({0xc0000b0d20, 0x0})
        /path/to/go/pkg/mod/github.com/rs/zerolog@v1.19.0/log.go:342 +0x2d
github.com/rs/zerolog.(*Event).msg(0xc0004a6c00, {0xc0000b0d20, 0x5f})
        /path/to/go/pkg/mod/github.com/rs/zerolog@v1.19.0/event.go:147 +0x2b8
github.com/rs/zerolog.(*Event).Msgf(0xc0004a6c00, {0x1738dc3, 0xc00038a3d0}, {0xc000063f98, 0xc00038a270, 0xc00038a3a8})
        /path/to/go/pkg/mod/github.com/rs/zerolog@v1.19.0/event.go:127 +0x4e
go.dedis.ch/dela/mino/proxy/http.(*HTTP).Listen(0xc0000aa3f0)
        /path/to/go/pkg/mod/go.dedis.ch/dela@v0.0.0-20211018150429-1fdbe35cd189/mino/proxy/http/mod.go:108 +0x3be
created by go.dedis.ch/dela/mino/proxy/http/controller.startAction.Execute
        /path/to/go/pkg/mod/go.dedis.ch/dela@v0.0.0-20211018150429-1fdbe35cd189/mino/proxy/http/controller/action.go:28 +0xf4
	
panic: failed to create conn '127.0.0.1:8081': listen tcp 127.0.0.1:8081: bind: address already in use

goroutine 282 [running]:
github.com/rs/zerolog.(*Logger).Panic.func1({0xc0000430e0, 0x0})
        /path/to/go/pkg/mod/github.com/rs/zerolog@v1.19.0/log.go:342 +0x2d
github.com/rs/zerolog.(*Event).msg(0xc000032fc0, {0xc0000430e0, 0x5f})
        /path/to/go/pkg/mod/github.com/rs/zerolog@v1.19.0/event.go:147 +0x2b8
github.com/rs/zerolog.(*Event).Msgf(0xc000032fc0, {0x171d6be, 0xc00003e4d0}, {0xc00009ef98, 0x0, 0xc00003e468})
        /path/to/go/pkg/mod/github.com/rs/zerolog@v1.19.0/event.go:127 +0x4e
go.dedis.ch/dela/mino/proxy/http.(*HTTP).Listen(0xc0001d6240)
        /path/to/go/pkg/mod/go.dedis.ch/dela@v0.0.0-20211018150429-1fdbe35cd189/mino/proxy/http/mod.go:108 +0x3be
created by go.dedis.ch/dela/mino/proxy/http/controller.startAction.Execute
        /path/to/go/pkg/mod/go.dedis.ch/dela@v0.0.0-20211018150429-1fdbe35cd189/mino/proxy/http/controller/action.go:28 +0xf4

We'd expect it not to crash.