[3062] Created a general CalloutHandle store

This change provides a template "CalloutHandle store" for use in the
DHCP servers.  It replaces server-specific methods and adds the ability
to clear the stored pointers.
[3063] First draft of a Hooks Framework maintenance manual
[3028] Hide the print_settings command

It is a testing command only, user is expected to examine configuration
by config show. So don't show it.
[3028] Get rid of the print_settings command completely

It was used in tests only. We may abuse the shutdown command for that.
[2983] Remaining v4 hooks implemented.
[3105] Add initialization of the ParserContext::hooks_libraries_.

This commit also changes the ParserContext copy constructor and assignment
operator to handle copying of the NULL shared pointers.
[2983] Unit-tests for buffer4_receive implemented.
[2983] Server now handles v4 packet without message type option correctly.
[2983] buffer4_receive tests are now passing.
[2983] Couple todos added regarding v4 packet validation
[master] Corrected module names in the ChangeLog entry 663.
[2983] buffer6_send tests implemented.
[2983] lease4_release tests implemented.
[master] Merge branch 'trac3105'
[2983] lease4_renew unit-tests implemented.
[2983] Minor comments corrections in v6 hooks tests
[3028] Check the shutdown is actually called
[3028] Remove outdated comment
[2983] Developer's Guide updated, ChangeLog proposal added.
[master] Merge branch 'trac3062'

Corrects a bug in Hooks code that was improperly
creating a new callout handle on every call, rather
than maintaining it throughout the context of the
packet being processed.