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And a minor cleanup in the code

And update doc and do a bit of style cleanup

And update doc and do a bit of style cleanup

Conflicts:
	doc/devel/mainpage.dox
	src/bin/dhcp4/ctrl_dhcp4_srv.h
	src/bin/dhcp4/dhcp4_srv.h
	src/bin/dhcp6/dhcp6.dox

Also modified the following files during the
resolution of the merge issues:
    src/bin/dhcp4/config_parser.cc
    src/bin/dhcp4/ctrl_dhcp4_srv.cc
    src/bin/dhcp4/dhcp4.dox
    src/bin/dhcp6/config_parser.cc
    src/bin/dhcp6/tests/dhcp6_srv_unittest.cc
    src/lib/dhcp/option_custom.h
    src/lib/dhcp/pkt4.h
    src/lib/dhcpsrv/database_backends.dox

There were some small changes in the behaviour of MasterLoader as a
result of review, this is update do ZoneLoader and its tests to
incorporate that.

Reviewed as part of #2378, but merged as a separate branch.

The Zone loader class (glue class between zone updater and MasterLoader).

Some of the code can be shared between the variants, saving some code.

After the rebase on top of working 2377. The most significant - don't
access updater if it doesn't exist.

There's no way to know the master file would be broken in the
constructor. So update the documentation to say it'd throw from the
load() and loadIncremental().

They are similar to the ones for copy mode. Should they be unified in
some way?

Not directly related, but it breaks tests of the ZoneLoader. The NSEC3
namespace and all RRSIGs were missing from the returned data.