Session-based nonces.

Previously nonces were stored in a global map which might not scale
well due to some issues:
1. The map uses the nonces as map keys. There were possible nonce
   collisions which required special handling.
2. The collision handling code was not thread safe. There were
   possible race conditions during concurrent modifications.
3. The map was shared and size limited. If there were a lot of
   users using the system, valid nonces could get pruned.
4. The map maps the nonces to client certificates. This limits
   the possible authentication methods that can be supported.

Now the code has been modified such that each user has a private map
in the user's session to store the nonces. Additional locking has been
implemented to protect against concurrent modifications. The map now
uses the target of the operation as the map key, eliminating possible
collisions and allowing the use of other authentication methods. Since
this is a private map, it's not affected by the number of users using
the system.

Ticket #474

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