[3400] Initial JSON file read support added in b10-dhcp6

- added --with-kea-config=JSON,BIND10 added to configure.ac
- JSON-file config backend added for b10-dhcp6
- Config file can now be read from a JSON file
[3400] Several improvements in JSON backend:

 - file read can now ignore # comments in file
 - there are JSON specific unit-tests now in src/bin/dhcp6
 - BIND10 backend specific tests moved to separate file
[3400] Documentation updated.
[3400] Example config file.
[3382] v4 Working with new PsqlBindArray

Initial use of PsqlBindArray, working for v4.
[3382] v6 working with new PsqlBindArray

Initial use of PsqlBindArray, working for v6.
[3382] Replaced remaining uses of PgSqlParam with PsqlBindArray

Replaced use of PgSqlParam with PsqlBindArray throughout.
Updated commentary and general clean up.
[3328] Removed use of PgSqlStatementBind

Unlike, MySQL, PostgreSQL does not return anything to the caller when
preparing an SQL statement, rather one uses its text name to execute it.
PgSqlStatementBind was an unecessary duplication of data already available
in tagged_statements.  This also resolved the unused member complaint from
cppcheck regarding TaggedStatement::index.

Also corrected two cppchecks in unit tests.
[3382] Replaced hard coded column types with constants

Replaced hard coded numbers for column types in tagged_statements[]
initialization.
Replaced test of error message text for duplicate key violations with
test of the SQL state code.  This is the recommended mechanism as
error text can be affected by localization and is subject to content
change.
[3382] A bit more clean up.

I removed todos on regarding use of binary of text.  I could not demonstrate
any appreciable difference between the two forms and binary results require
more manual conversion. Performance should be examined as a separte effort.
[trac3400] ChangeLog updated.
[3398] The result of configuration design discussion added
[3398] Several Doxygen warnings removed.
[3398] Rather important file has been omitted. :)
[3398] Miscellaneous corrections and edits as part of review
[3400] Changes after review:

- config-reload command added
- config and command handlers cleaned up
- files renamed
- Majority of the backend code is now common
- Added unit-test for config-reload command
[3400] Skeleton for Bundy backend added.
[3268] Treat top-level scalars as a group of globals parameters

Restructured DCfgMgrBase to group the top level elements in a configuration
into scalars (strings, bools, ints, etc...) and objects (maps, lists, etc),
and parse the scalars first, then objects.  This permits the top level
scalars to be treated as a group of global parameters that are parsed first.

Ordered parsing is now relegated to only object elements. Scalars are parsed
first before any objects.

Also added the ability to reset config manager's context and rather than
than starting configuration parsing with a copy of the current context, it
starts with an empty context.

Modified unit tests accordingly.
[3268] Added container class for D2 global parameters

Added d2::D2Params to act as container class for D2 global parameters.
This permits them to be converted from their context parameter form during
configuration parsing.
[3268] Modified D2CfgMgr and spec file

Added D2CfgMgr::buildParams() method to support the new parameters first
processing.

Removed support D2's global "interface" parameter, and added three more:
 - dns_server_timeout
 - ncr_protocol
 - ncr_format