service/dap: set seq field in responses

[polinasok]

When studying vscode-go logs, I saw that seq was only used for requests and request_seq was used for responses while seq always stayed 0. For example:

To client: {"seq":0,"type":"response","request_seq":1,"command":"initialize","success":true,"body":{"supportsConditionalBreakpoints":true,"supportsConfigurationDoneRequest":true,"supportsSetVariable":true}}
To client: {"seq":0,"type":"event","event":"output","body":{"category":"stdout","output":"API server listening at: 127.0.0.1:38993\n"}}
To client: {"seq":0,"type":"event","event":"initialized"}
To client: {"seq":0,"type":"response","request_seq":2,"command":"launch","success":true}

It turns out that this is an artifact of a bug in the parent code (microsoft/vscode-debugadapter-node#239) and in fact increasing seq numbrers are used. It doesn't seem like this is hurting anything, so this is likely just for bookkeeping on our end.