DAOS-16653 pool: Batch crt events (#15230)

* DAOS-16653 pool: Batch crt events

When multiple engines become unavailable around the same time, if a pool
cannot tolerate the unavailability of those engines, it is sometimes
desired that the pool would not exclude any of the engines. Hence, this
patch introduces a CaRT event delay, tunable via the server-side
environment variable, CRT_EVENT_DELAY, so that the events signaling the
unavailability of those engines will be handled in hopefully one batch,
giving pool_svc_update_map_internal a chance to reject the pool map
update based on the RF check.

When the RF check rejects a pool map change, we should revisit the
corresponding events later, rather than simply throwing them away. This
patch improves this case by returning the events back to the event
queue, and pause the queue handling until next new event or pool map
update.

  - Introduce event sets: pool_svc_event_set. Now the event queue can be
    simplified to just one event set.

  - Add the ability to pause and resume the event handling: pse_paused.

  - Track the time when the latest event was queued: pse_time.


Signed-off-by: Li Wei <wei.g.li@intel.com>
Required-githooks: true

docs/admin/env_variables.md

@@ -44,6 +44,7 @@ Environment variables in this section only apply to the server side.
 |DAOS\_MD\_CAP         |Size of a metadata pmem pool/file in MBs. INTEGER. Default to 128 MB.|
 |DAOS\_START\_POOL\_SVC|Determines whether to start existing pool services when starting a daos\_server. BOOL. Default to true.|
 |CRT\_DISABLE\_MEM\_PIN|Disable memory pinning workaround on a server side. BOOL. Default to 0.|
+|CRT\_EVENT\_DELAY|Delay in seconds before handling each CaRT event. INTEGER. Default to 10 s. A longer delay enables batching of successive CaRT events, leading to fewer pool map changes when multiple engines become unavailable at around the same time.|
 |DAOS\_SCHED\_PRIO\_DISABLED|Disable server ULT prioritizing. BOOL. Default to 0.|
 |DAOS\_SCHED\_RELAX\_MODE|The mode of CPU relaxing on idle. "disabled":disable relaxing; "net":wait on network request for INTVL; "sleep":sleep for INTVL. STRING. Default to "net"|
 |DAOS\_SCHED\_RELAX\_INTVL|CPU relax interval in milliseconds. INTEGER. Default to 1 ms.|