It's meant to have a special status in the analysis, whereby latencies
from important marks to all their enclosing BEGIN ({) and END (})
markings will be measured.

-) Fixed a few more (much more rare) race conditions on shutdown.
-) Fixed a degenerate case if we create a server but never start it.

Fixing a very edge case where the tcp_client can crash due to race conditions on connection abortion.

-) tcp client and server should no longer starve waiting on orphans
-) proper server shutdown sequence to prevent use-after-free.

By the standard, std::function can only capture class functions, and not
class methods. For some reasons, this works under gcc however. Visual
Studio rejects it. std::mem_fn is proper template to use.