MINOR: server: move the lock inside srv_add_idle()

MINOR: server: move the lock inside srv_add_idle()

Almost all callers of _srv_add_idle() lock the list then call the
function. It's not the most efficient and it requires some care from
the caller to take care of that lock. Let's change this a little bit by
having srv_add_idle() that takes the lock and calls _srv_add_idle() that
is now inlined. This way callers don't have to handle the lock themselves
anymore, and the lock is only taken around the sensitive parts, not the
function call+return.

Interestingly, perf tests show a small perf increase from 2.28-2.32M RPS
to 2.32-2.37M RPS on a 128-thread system.