[2751] Tests for the subtract method
[2274] fix typo and change column order of label in initial statistics counter checking
[2274] adjust expected counters' value according to build bot

Because behavior is changed on the build bot, increase values of expected
counters on the following scenarios:

  Scenario: Handle incoming notify (XFR request rejected)
  Scenario: Handle incoming notify (XFR request rejected in IPv4)

See also for details:
  https://lists.isc.org/pipermail/bind10-dev/2013-July/004769.html
[2274] check statistics counters for Xfrin in lettuce test

explicitly check new statistics counters for Xfrin are just zero (incremented
and decremented) after zone xferring in lettuce test
[2274] document in the man page about statistics counters introduced in Xfrin

  ixfr_running
  axfr_running
  soa_in_progress
[2300] check statistics socket counters for Xfrin in lettuce test
[2300] document in the man page about statistics socket counters introduced in Xfrin

  open
  openfail
  close
  connfail
  conn
  senderr
  recverr
[3084] Addressed review comments.
[3084] Link libb10-dhcp-ddns with cc to prevent linking issue on Ubuntu 12.
[master] mention more local asio modifications
[3113] Fix hooks unit test failures when --enable-static-link is used.
[2750] Add code comments
[2750] Add comments for rebalancing code in remove()
[2750] Simplify condition

It has the same effect, but is simpler to read.
[2750] Remove redundant argument
[2750] Recompute sibling after rotations
[2750] Update some checks
[master] Merge branch 'trac3084'
[master] Added ChangeLog entry for #3084.
[master] Second merge of branch 'trac3084'

By mistake, the previous merge was missing the changes which addressed
review comments.