MEDIUM: checks: avoid accumulating TIME_WAITs during checks

MEDIUM: checks: avoid accumulating TIME_WAITs during checks

Some checks which do not induce a close from the server accumulate
local TIME_WAIT sockets because they're cleanly shut down. Typically
TCP probes cause this. This is very problematic when there are many
servers, when the checks are fast or when local source ports are rare.

So now we'll disable lingering on the socket instead of sending a
shutdown. Before doing this we try to drain any possibly pending data.
That way we avoid sending an RST when the server has closed first.

This change means that some servers will see more RSTs, but this is
needed to avoid local source port starvation.