[2883] remove _statistics class attribute

and define statistics data and spec inside instance scope
[2883] spec_file_name argument of Counters class is reqiured
[2883] create a counters object once in the test class

and pass to each class via class argument
[2883] remove TestDummyNotifyOut

TestDummyXfroutServer class contains similar tests
[2883] no need to reload module and do tearDown in test classes

The counters object doesn't have a static statistics object
[2883] additional statistics tests (not directory related)
[2883] add counters argument into each related classes

and create a Counters object in XfroutServer class
[2883] add counters as a keyword argument into NotifyOut
[2883] add a test specfile for using a counters object in TestNotifyOut class
[2883] add counters arguments into XfrinConnection class and its related methods
[2883] no need to clear all counters before each test
[2781] correct inappropriate condition (not related to this ticket)

correct inappropriate condition in the internal function and add
related tests, and update the copyright years
[2781] unify similar outputs of STATS_RECEIVED_INVALID_STATISTICS_DATA

This change is rather for #2944 (duplicate log ID in stats).
[2781] break do_polling() into some internal methods:

_get_multi_module_list()
_query_statistics()
_collect_statistics()
_refresh_statistics()
[2781] add test_refresh_statistics_broken_statistics_data()

add test_refresh_statistics_broken_statistics_data() by reusing test_polling2()
[2781] import create_answer instead of its shortcut
[2781] make MyStats.__seq visible for testing purpose
[2939] Check for python sqlite3 module
[2781] add new tests for each method broken from do_polling()

  test_polling_update_lasttime_poll()
  test_get_multi_module_list()
  test_get_multi_module_list_rpcrecipientmissing()
  test_get_multi_module_list_rpcerror()
  test_query_statistics()
  test_collect_statistics()
  test_collect_statistics_nodata()
  test_collect_statistics_nonzero_rcode()
  test_collect_statistics_sessiontimeout()
  test_refresh_statistics()
[2781] modify mock classes in test_utils.py

add check_command_without_recvmsg() into MyModuleCCSession and add default
values into args of __group_recvmsg(). These changes are for referring and
executing from the unit tests.