Add time quota in checking global objects

[liufengyun]

Make global object checker default to best-effort mode

• In best-effort mode, the checker tries to be fast, useful and unobtrusive.
• In the aggressive mode, the checker tries to be sound by spending more check time and produce more verbose warnings.

In both modes, there will be a worst-case performance guarantee based on a quota on the number of method calls in initializing a global object.