ITS#7022 NSS_Init* functions are not thread safe

ITS#7022 NSS_Init* functions are not thread safe

The NSS_InitContext et. al, and their corresponding shutdown functions,
are not thread safe.  There can only be one thread at a time calling
these functions.  Protect the calls with a mutex.  Create the mutex
using a PR_CallOnce to ensure that the mutex is only created once and
not used before created.  Move the registration of the nss shutdown
callback to also use a PR_CallOnce.  Removed the call to
SSL_ClearSessionCache() because it is always called at shutdown, and we must
not call it more than once.