Adds use of lease type as a discriminator in DHCP6 lease queries in MySQL
back end.

Minor changes to logging messages, unit tests, and commentary.

Don't crash on startup because message definition module is missing,
install it like the rest.

Conflicts:
	ChangeLog

Support for RdataSet::subtract.

 - lease_type_ renamed to pool_type_
 - exception message clarified
 - prefix lengths comment updated
 - couple EXPECT_NO_THROWs added to subnet unit-tests

Generic traceback handling in python.

Conflicts:
	src/bin/memmgr/memmgr.py.in

The UNHANDLED_EXCEPTION is renamed to PYTHON_UNHANDLED_EXCEPTION,
because only python code produces this one.

Added support for lease type discrimination in IPv6 lease
operations in MySQL backend used by DHCP6, dhcpsrv/mysql_lease_mgr

Conflicts:
	ChangeLog

This apparently happens when gtest-1.5 is in use. Patch was reviewed by
Thomas Markwalder.