JAMES-3196 Avoid MDC costs when not needed

[chibenwa]

Before we pay the cost of building MDC for each line of the message, which is useless upon copying the content (won't log in the enclosing context).

Significant performance gains can be achieved by only wrapping the final operation in the MDC:

Run locally, to generate the load:

time smtp-source -A -C100 -r 1 -l 500000 -m 5000 -s 20 -d -c -f any@other.com -M other.com -t bob@localhost localhost:25

Which sends 5000 emails of 500KB each...

Before it takes 2 min 30, and the SMTP receival goes at the same speed than the dequeuing

After, it takes 1min40, ~1min to enqueue and 40 more seconds so complete the dequeue...