fix: max_active_session does not work for http handler.

[youngsofun]

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Summary

Summary about this PR

• Closes #issue

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This change is 

[youngsofun]

1. new impl not longer add dummy session to active sessions unless it is later upgraded. otherwise multi concurrent dummy sessions will occupy the slots for normal sessions, especially make it hard to test .
2. add tests
3. test for mysql handler changed. now allow num_sessions == max_sessions, and we always check <= before add.

@zhang2014 @ariesdevil