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