Fix for issues 3255, 3265, and 3269:

Issue 3255: Added ds-cfg-max-allowed-client-connections configuration parameter to prevent using up all file descriptors. By default the server will still allow unlimited number of connections just like before. The user will have to adjust this for their system. This might be automatically set for the OS in the future.

Issue 3265: Subtree deletes and mod DN operations will now always use a JE transaction throughout the entire operation. This makes sure the operations are always executed atomically.

Issue 3269: Added checks for canceled operations in JE backend code.