Instrument the SMTP code using runtime/trace

runtime/trace together with 'go tool trace' provides extremely powerful
tooling for performance (latency) analysis. Since maddy prides itself on
being "optimized for concurrency", it is a good idea to actually live up
to this promise.

Closes #144. No need to reinvent the wheel. The original issue
proposed a solution to use in production to detect "performance
anomalies", it is possible to use runtime/trace in production too, but
the corresponding flag to enable profiler endpoint is hidden behind the
'debugflags' build tag at the moment.

For SMTP code, the basic latency information can be obtained from
regular logs since they include timestamps with millisecond granularity.
After the issue is apparent, it is possible to deploy the server
executable compiled with tracing support and obtain more information

... Also add missing context.Context arguments to smtpconn.C.

internal/modify/dkim/dkim.go

@@ -7,6 +7,7 @@ import (
 	"io"
 	"net/mail"
 	"path/filepath"
+	"runtime/trace"
 	"strings"
 	"time"
 
@@ -337,6 +338,8 @@ func (s state) RewriteRcpt(ctx context.Context, rcptTo string) (string, error) {
 }
 
 func (s state) RewriteBody(ctx context.Context, h *textproto.Header, body buffer.Buffer) error {
+	defer trace.StartRegion(ctx, "sign_dkim/RewriteBody").End()
+
 	var authUser string
 	if s.meta.Conn != nil {
 		authUser = s.meta.Conn.AuthUser