- Introduced dependency between 'checksum_variables' and their
  corresponding '%_diffs_before_sync' variable. Checksum variables being
  'false' implies a '0' value. Variable dependencies need to be
  introduced in the handler for both variables, since order can't be
  assumed.
- Added missing sets statements for 'GloProxyCluster' variables when
  updating '%_diffs_before_sync'.
- Fixed typo in 'cluster_ldap_variables_diffs_before_sync' handling,
  creating unintentional checksum resets.

- Improve test doc related to module checksums sync.
- Add two way sync testing between primary and secondary.
- Add support for testing deprecated 'checksum_%' variables.

…ksum_vars_fix