[Refactor][core]Data inflation when using postgres

[narrowizard]

What and why to refactor

As a software engineer, I am using devlake to collect developer data since long time ago. Recently we find that the _devlake_subtasks table occurs a data inflation. It used 750MB after a week from we upgrading devlake v1.0, but there were only 1000 records in the table.

Describe the solution you'd like

Solutions from @klesh

1. Reduce the update rate to _devlake_subtasks table when collect data.
2. Store progress info in memory firstly and write it to db in a fixed rate.

Related issues

No

Additional context

• The issue was introduced in v1.0.
• We are using postgres.

[narrowizard]

After reading some code,I organizaed a table to show when we updading the progress of sub tasks.

Trigger timing	Times	Code
Sub task started	Once a sub task	https://github.com/apache/incubator-devlake/blob/98bc98bc9b62108834cfa10b554d708b64c1e932/backend/core/runner/run_task.go#L308-L314
Mannually called by sub task context	Unknown(decide by plugin implement)	https://github.com/apache/incubator-devlake/blob/98bc98bc9b62108834cfa10b554d708b64c1e932/backend/impls/context/default_exec_context.go#L68-L97 https://github.com/apache/incubator-devlake/blob/98bc98bc9b62108834cfa10b554d708b64c1e932/backend/impls/context/default_subtask_context.go#L37-L53
Mannually called by task context	Unknown	https://github.com/apache/incubator-devlake/blob/98bc98bc9b62108834cfa10b554d708b64c1e932/backend/impls/context/default_task_context.go#L40-L49

Seems that the progress updating calling is not always under control, so we need to decrease db operation automatically.