BUG/MEDIUM: stats-file: detect and fix inconsistent shared clock when resuming from...

BUG/MEDIUM: stats-file: detect and fix inconsistent shared clock when resuming from shm-stats-file

When leveraging shm-stats-file, global_now_ms and global_now_ns are stored
(and thus shared) inside the shared map, so that all co-processes share
the same clock source.

Since the global_now_{ns,ms} clocks are derived from now_ns, and given
that now_ns is a monotonic clock (hence inconsistent from one host to
another or reset after reboot) special care must be taken to detect
situations where the clock stored in the shared map is inconsistent
with the one from the local process during startup, and cannot be
relied upon anymore. A common situation where the current implementation
fails is resuming from a shared file after reboot: the global_now_ns stored
in the shm-stats-file will be greater than the local now_ns after reboot,
and applying the shared offset doesn't help since it was only relevant to
processes prior to rebooting. Haproxy's clock code doesn't expect that
(once the now offset is applied) global_now_ns > now_ns, and it creates
ambiguous situation where the clock computations (both haproxy oriented
and shm-stats-file oriented) are broken.

To fix the issue, when we detect that the clock stored in the shm is
off by more than SHM_STATS_FILE_HEARTBEAT_TIMEOUT (60s) from the
local now_ns, since this situation is not supposed to happen in normal
environment on the host, we assume that the shm file was previously used
on a different system (or that the current host rebooted).

In this case, we perform a manually adjustment of the now offset so that
the monotonic clock from the current host is consistent again with the
global_now_ns stored in the file. Doing so we can ensure that clock-
dependent objects (such as freq_counters) stored within the map will keep
working as if we just (re)started where we left off when the last process
stopped updating the map.

Normally it is not expected that we update the now offset stored in the
map once the map was already created (because of concurrent accesses to
the file when multiple processes are attached to it), but in this specific
case, we know we are the first process on this host to start working
(again) on the file, thus we update the offset as if we created the
file ourself, while keeping existing content.

It should be backported in 3.3