Multi-threading support with object pool

[Crypt32]

This PR contains a potential fix for issue #2.

In short, CertServer property in CertPolicyBase and CertExitBase is not thread-safe and it appears that VerifyRequest and Notify methods are called by CA asynchronously, which results in exception and CA service crash.

I've expressed my concerns about possible fixes that would:

• not overcomplicate policy and exit module development
• protect module from resource overconsumption
• reduce managed/unmanaged resource allocation and cleanup
• boost performance in multi-threading

This PR comes with a object pool of CertServerModule instances. The idea is inspired by Microsoft CLM policy module. Pool size is fixed and have some limits. Upper size limit is 63. At any given time, no more than 63 threads (or requests) can be processed in parallel. If there are more parallel calls, they are put in queue until any currently running thread finish. Entire pool is pre-created during policy module initialization, so extra resources are allocated only once.

I think it should greatly improve policy module performance over single-threded (with locks on single CertServerModule instance). However, due to the way how previous approach was implemented, it is not possible to fix race condition issue without introducing breaking change. These changes include:

• CertPolicyBase.VerifyRequest(String, Int32, Int32, Int32) and CertExitBase.Notify(ExitEvents, Int32) methods (that implement required interface) are no longer virtual and do not allow override.
• Instead, they call new abstract methods CertPolicyBase.VerifyRequest(CertServerModule, PolicyModuleAction, Boolean) and CertExitBase.Notify(CertServerModule, ExitEvents, Int32) respectively, where first parameter provides a thread-safe instance of CertServerModule.

This framework automatically acquire (and wait if necessary) and return object to the pool.