The public methods of all CertStoreSpi objects must be
thread-safe. That is, multiple threads may concurrently invoke these
methods on a single CertStoreSpi object (or more than one)
with no ill effects. This allows a CertPathBuilder to search
for a CRL while simultaneously searching for further certificates, for
Simple CertStoreSpi implementations will probably ensure
thread safety by adding a synchronized keyword to their
engineGetCertificates and engineGetCRLs methods.
More sophisticated ones may allow truly concurrent access.