Instances of this class need not be protected against concurrent
access from multiple threads. Threads that need to access a single
CertPathValidatorSpi instance concurrently should synchronize
amongst themselves and provide the necessary locking before calling the
wrapping CertPathValidator object.
However, implementations of CertPathValidatorSpi may still
encounter concurrency issues, since multiple threads each
manipulating a different CertPathValidatorSpi instance need not