Add a property to limit user instance num.

[liutang123]

Proposed changes

Add a property to limit user instance num.

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)
• Code refactor (Modify the code structure, format the code, etc...)

Checklist

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• I have created an issue on (Fix [Feature] Add user property to limit the instance num #6158) and described the bug/feature there in detail
• Compiling and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• If these changes need document changes, I have updated the document
• Any dependent changes have been merged

Further comments

[morningman]

maxQueryInstances can be saved in ConnectionContext when connection established, so that no need to calculate this variables at every registerInstances()

[liutang123]

If save maxQueryInstances in ConnectionContext , we can not change it after ConnectionContext created.
getMaxQueryInstances will try to obtain an read lock in PlaoAuth. I think the cost should be very small, because it is a situation of reading more and writing less.

However, we can update parameters and disconnect all links to make parameters take effect.

[morningman]

I made a test, in high concurrency scenarios, QPS has a drop of about 10% (from 11000/s to 10000/s)
But I think its OK for most cases.

[morningman]

We can create a new class and put all user property in it. And using json to serialize it. So that we don't need to update the meta version each time we add a new property in future.

[morningman]

LGTM