crypto: use NSS_InitContext/NSS_ShutdownContex to avoid memory leak

crypto: use NSS_InitContext/NSS_ShutdownContex to avoid memory leak

Switched to context-aware NSS init/shutdown functions to avoid conflicts
with parent application.  Use a reference counter to properly shutdown the
NSS crypto library when the last CephContext is destroyed.  This avoids
memory leaks with the NSS library from users of librados.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>