Add a knob to allow configuring SO_LINGER on all opened sockets

In a situation where clients are particularly short lived on a host, there will be many connections opened repetitively to the same FDB processes and ports. This means the host will rapidly accumulate sockets left in TIME_WAIT, and for how long is at the discretion of the kernel configuration.

It'd be nice to have finer grained control over how long sockets will linger in TIME_WAIT. Asio supports this as a socket option, but I don't see anything in FDB that allows configuring it. (And an integer would be nicer, instead of just 0 or unset.)

No file or test is named. Start by locating socket creation and existing Asio socket-option handling, then trace how configuration reaches all opened sockets; done means an integer setting controls SO_LINGER consistently and its behavior is verified.

Written by the indexing model from the issue text.