`@async` instead of `Threads.@spawn` in `listenloop`

[theogf]

Hi!

I was wondering if there is any other reason than legacy for using @async in the listenloop under Servers.jl ?

HTTP.jl/src/Servers.jl

Lines 399 to 407 in 537f50c

	conn.host, conn.port = listener.hostname, listener.hostport
	@async try
	handle_connection(f, conn, listener, readtimeout, access_log, verbose)
	finally
	# handle_connection is in charge of closing the underlying io
	Base.@lock conns_lock delete!(conns, conn)
	Base.release(sem)
	end
	catch e

It feels like we should be able to make the task non-sticky by using Threads.@spawn instead.