Update memory tracker periodically with cgroup memory usage

[rschu1ze]

When the system runs out of memory (as determined by system-wide settings or cgroups memory thresholds), the OOM killer stops memory-intensive processes. Due to its nature, ClickHouse will often be the victim. To avoid this, the upper limit of the (global) memory tracker in ClickHouse can be configured to a value smaller than the "kill threshold", see setting max_server_memory_usage_ram_ratio.

There are some problems with this approach:

• The maximum memory threshold of the processes's cgroup can be changed (increased, decreased) while ClickHouse runs. An earlier fix (Respect cgroup memory limit when reloading configuration #53455) ensured that new memory limits were at least considered when ClickHouse's configuration changes (a follow-up fix Consider nested cgroup memory limits for the total memory tracker #59435 resolved problems with nested cgroups). Of course, users need to make real or dummy configuration changes for this to happen and it may already be too late at this point.

• The memory tracking in ClickHouse is not 100% precise. For example, ClickHouse may think it uses less memory than the OS thinks. In that case, there is a risk of getting OOM-killed. Or the other way round, ClickHouse produces OOMs even if there is sufficient memory.

This PR tries to address this by reading the current memory consumption as seen by cgroups periodically and running two possible actions:

• if actual memory consumption (as read from cgroups) exceeds a soft threshold, purge jemalloc preallocated arenas. This should bring the OS's and ClickHouse's view of the memory consumption closer together.
• if a hard threshold is exceeded, adjust (usually lower) the hard limit in the memory tracker to the exceeded threshold. This increases the chance for OOMs in ClickHouse but decreases the risk of getting OOM killed.

The read interval (setting cgroups_memory_usage_observer_wait_time) is 15 sec by default and it can be reduced to 1 sec if needed. 0 means disabled. The parameter cannot be changed after database start.

The hard and soft limits are controlled by parameters cgroup_memory_watcher_hard_limit_ratio and cgroup_memory_watcher_soft_limit_ratio (defaults: 0.95, 0.9). They each refer to the initial maximum allowed global memory consumption computed as total system memory x max_server_memory_usage_to_ram_ratio. Both parameters can be changed at runtime.

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