[services-ng] add xlog enforcement for postgresql service

1. Add a periodical timer to check the xlog file number, interval is 1 second
    - non-wardenized case: only write a warnning in the log
    - wardenized case:
      1) if xlog_file_num in [ 3 * checkpoint_segments + 2, 4*checkpoint_segments + 2), 
          issue a force checkpoint, try to delete unneeded segment files
      2) if xlog_file_num in [ 4*checkpoint_segments + 2, ~) after a continous 5 (xlog_enforce_tolerance) times check, 
          terminate the connections from applications (won't terminate the super user and default users)
    - note:
          - only for oss postgresql (checkpoint_segments_max is not provided.) and plan  
            are sensitive to storage overhead for xlog files
          - this is a trade-off of performance and storage
            provide more storage for xlog is preferred.
          - this enforcer could be disabled
          - xlog_enforce_tolerance could be tuned.
2. Varz will include the xlog file number information