### What problem does this PR solve?

## Cause of the bug:
During the execution process, due to improper use of trio
CapacityLimiter, the configuration parameter MAX_CONCURRENT_TASKS is
invalid, causing the executor to take out a large number of tasks from
the Redis queue at one time.

This behavior will cause the task executor to occupy too much memory and
be killed by the OS when a large number of tasks exist at the same time.
As a result, all executing tasks are suspended.

## Fix:
Added the task_manager method to the entry of /rag/svr/task_executor.py
to make CapacityLimiter effective. Deleted the invalid async with
statement.

## Fix result:
After testing, the task executor execution meets expectations, that is:
concurrent execution of up to $MAX_CONCURRENT_TASKS tasks.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):