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After merging, distcheck revealed a few minor issues with logging
files and unit test data files. These were corrected.

Added doxygen throughout, cleaned up unit tests.  Removed CLIENT_ID type
as it was unnecessary.

Interrim checkin. Use of isc::logging has been disabled.  Initializing
the dictionary (MessageInitializer::loadDictionary) cores on Debian. This
has something to do with the libary being opened and closed without calling
load during Kea configuration parsing and the later reopening the library.
See trac#3198.

Replaced couts with isc::log mechanisms in user_chk DHCP hook library.

Added initial callout functions and ability to load the user_chk hooks library.

Added UserFile derivation of UserDataSource which allows the UserRegistry
to be populated from a JSON formatted file.

Adds new source tree branch, bind10/src/hooks, for maintaining
custom hooks libraries.  Adds bare bones implementation of UserRegistry
for use in custom DHCP hooks library.