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This change was suggested in the review.

Previously, the sockets were closed just before opening a new set of
sockets, rather than before reconfiguring the IfaceMgr. Some steps of the
IfaceMgr reconfiguration failed when sockets were still open, which
effectively led to inability to reconfigure the running server.

Conflicts:
	src/lib/asiolink/io_address.h
	src/lib/asiolink/tests/io_address_unittest.cc

 - IOAddress::increase() simplified
 - getLeasesCount renamed to getPoolCapacity
 - removed several intermediate variables

 - increaseAddress renamed to increase
 - check that addrsInRange throw when max < min
 - C++ style casts, consts added
 - check that a can be subtracted from b, when a>b
 - documentation for IOAddress::increase, subtract improved
 - couple unnecessary intermediate objects removed