internal/process: improve windows shutdown behaviour (#808)

Add Windows specific shutdown code paths so stopping of child processes
is more reliable:

- stopping llama-swap won't leave behind any child processes it created
- uses Job Objects in Windows so the whole llama-swap tree is closed by
the os
- add procCtx to baseRouter. It replaces shutdownCtx as a signal for
managing lifetime state.
- shutdownCtx is only used by the router to stop handling new requests
during shutdown
- improve debug logging to make it easier to trace source of issues

Fixes #804
Updates #807

internal/router/matrix.go

@@ -31,9 +31,10 @@ func NewMatrix(conf config.Config, proxylog, upstreamlog *logmon.Monitor) (*Matr
 
 	for mid, modelCfg := range conf.Models {
 		procLog := logmon.NewWriter(upstreamlog)
-		p, err := process.New(base.shutdownCtx, mid, modelCfg, procLog, proxylog)
+		p, err := process.New(base.procCtx, mid, modelCfg, procLog, proxylog)
 		if err != nil {
 			base.shutdownFn()
+			base.procCancel()
 			return nil, fmt.Errorf("creating process for %q: %w", mid, err)
 		}
 		processes[mid] = p