Fixed a crash caused by a race condition in LLRefCount.

Reason:
secapiSSLCertVerifyCallback() seems to be called simultaneously by multiple threads,
which causes a race condition in LLRefCount::ref/unref() methods.
The reference counter in LLSecAPIBasicHandler::mStore goes to zero, and the object gets destroyed.

Fix:
Derive LLCertificateStore from LLThreadSafeRefCount instead of LLRefCount,
which should fix the race condition.

Note:
The LLThreadSafeRefCount constructor is private, so we have to wrap instances of the class with LLPointer.