BUG/MEDIUM: listener: Fix race condition when updating the global mngmt task

BUG/MEDIUM: listener: Fix race condition when updating the global mngmt task

It is pretty similar to fbb934da90 ("BUG/MEDIUM: stick-table: fix a race
condition when updating the expiration task"). When the global management
task is running, at the end of its process function, it resets the expire
date by setting it to TICK_ETERNITY. In same time, a listener may reach a
global limit and decides to schedule the task. Thus it is possible to queue
the task and trigger the BUG_ON() on the expire date because its value was
set to TICK_ETERNITY in the means time:

  FATAL: bug condition "task->expire == 0" matched at src/task.c:310
    call trace(12):
    |       0x662de8 [b8 01 00 00 00 c6 00 00]: __task_queue+0xc7/0x11e
    |       0x63b03f [48 b8 04 00 00 00 05 00]: main+0x2535e
    |       0x63ed1a [e9 d2 fd ff ff 48 8b 45]: listener_accept+0xf72/0xfda
    |       0x6a36d3 [eb 01 90 c9 c3 55 48 89]: sock_accept_iocb+0x82/0x87
    |       0x6af22f [48 8b 05 ca f9 13 00 8b]: fd_update_events+0x35a/0x497
    |       0x42a7a8 [89 45 d8 83 7d d8 02 75]: main-0x1eb539
    |       0x6158fb [48 8b 05 e6 06 1c 00 64]: run_poll_loop+0x2e7/0x319
    |       0x615b6c [48 8b 05 ed 65 1d 00 48]: main-0x175
    | 0x7ffff775bded [e9 69 fe ff ff 48 8b 4c]: libc:+0x8cded
    | 0x7ffff77e1370 [48 89 c7 b8 3c 00 00 00]: libc:+0x112370

To fix the bug, a RW lock is introduced. It is used to fix the race
condition. A read lock is taken when the task is scheduled, in
listener_accpet() and a write lock is used at the end of process function to
set the expire date to TICK_ETERNITY. This lock should not be used very
often and most of time by "readers". So, the impact should be really
limited.

This patch should fix the issue #1930. It must be backported as far as 1.8
with some cautions because the code has evolved a lot since then.