feat: raise load_from_private_log priority from LOW to COMMON

[ninsmiracle]

What problem does this PR solve?

#2183

What is changed and how does it work?

Raise the priority of load_from_private_log from LOW to COMMON.

Performance Testing

I do some test cases as following:
In these test cases, I first wrote 8k QPS of write traffic to the master cluster (this is the traffic that my test cluster will not generate dup log backlogs) to verify the effect of the priority modification. Then I wrote 20k QPS of write traffic to the master cluster (this is the traffic that my test cluster will generate a certain degree of dup log backlogs) to verify the effect of the priority modification.

load/ship task priority	load_from_private_log task priority	qps	duplicate_log_batch_bytes	plog Maximum backlog	Master cluster write delay p99	master/slave dup delay
LOW	LOW	8K	4096	9k	1ms	p95 127ms、p99 27473ms
LOW	COMMON	8K	4096	200	1ms	p95 101ms、p99 109ms
HIGH	HIGH	8K	4096	150	1ms	p95 107ms、p99 115ms
LOW	LOW	20K	4096	61K	1.5ms	p95 139ms、p99 20506ms
LOW	COMMON	20K	4096	42K	1.5ms	p95 126ms、p99 18127ms
LOW	COMMON	20K	0	Continue to increase over time	1.5ms	95 10618ms、p99 303519ms

As you see , change the priority from LOW to COMMON of load_from_private_log will not t increase the online delay. And priority from LOW to HIGH is no benefit for further speeding up duplication.

So based on the above experimental conclusions, I think this issues' argument is valid.

[empiredan]

load_from_private_log_level seems to a configuration for the log level of something. Also, it does not show the relationship to duplication. In addition, this configuration is essentially a task code specifying a class of tasks running on the specific thread pool. We could use dup_load_plog_task to show a configurable task for incremental loading from private logs while duplicating.

Suggested change

	load_from_private_log_level,
	dup_load_plog_task,

[empiredan]

Suggested change

	DSN_TAG_VARIABLE(load_from_private_log_level, FT_MUTABLE);
	DSN_TAG_VARIABLE(dup_load_plog_task, FT_MUTABLE);

[empiredan]

Suggested change

	const dsn::task_code load_from_private_log_level =
	dsn::task_code::try_get(FLAGS_load_from_private_log_level, LPC_REPLICATION_LONG_LOW);
	LOG_ERROR_PREFIX("unexpected load_from_private_log_level ({}), put it to default value "
	"LPC_REPLICATION_LONG_LOW",
	load_from_private_log_level);
	fork(*_load_private, load_from_private_log_level, 0).link(*_ship);
	auto dup_load_plog_task =
	dsn::task_code::try_get(FLAGS_dup_load_plog_task, TASK_CODE_INVALID);
	if (dup_load_plog_task == TASK_CODE_INVALID) {
	dup_load_plog_task = LPC_REPLICATION_LONG_LOW;
	LOG_ERROR_PREFIX("invalid dup_load_plog_task ({}), set it to LPC_REPLICATION_LONG_LOW",
	FLAGS_dup_load_plog_task);
	}
	fork(*_load_private, dup_load_plog_task, 0).link(*_ship);

[empiredan]

Suggested change

	"The level of load_from_private_log when doing a duplication.Be false means the "
	"task level of replaing plog is low, otherwise the task level is common (We do "
	"not recommend high level)");
	"The task code for incremental loading from private logs while duplicating. Tasks with "
	"TASK_PRIORITY_HIGH are not recommended.");