fix deprecated system variables

[xxxmakotoxxx]

SQL_MAX_JOIN_SIZE is deprecated system variable.
use MAX_JOIN_SIZE instead.
see: https://dev.mysql.com/doc/refman/5.6/en/mysql-nutshell.html#mysql-nutshell-removals

so error in t/001_mysql_sharding_client.t

$ prove t/001_mysql_sharding_client.t
t/001_mysql_sharding_client.t .. 10/?
    #   Failed test 'connect DBD::mysql::db do failed: Unknown system variable 'SQL_MAX_JOIN_SIZE' at /path/to/.plenv/versions/5.20.3/lib/perl5/site_perl/5.20.3/MySQL/Sharding/Client.pm line 141.
    # '
    #   at t/001_mysql_sharding_client.t line 113.

    #   Failed test 'prepare insert OK db1'
    #   at t/001_mysql_sharding_client.t line 124.
    # Looks like you failed 2 tests of 2.

#   Failed test 'pre commands'
#   at t/001_mysql_sharding_client.t line 132.
Can't use an undefined value as a HASH reference at t/001_mysql_sharding_client.t line 125.
# Tests were run but no plan was declared and done_testing() was not seen.
# Looks like your test exited with 102 just after 14.
t/001_mysql_sharding_client.t .. Dubious, test returned 102 (wstat 26112, 0x6600)
Failed 1/14 subtests

Test Summary Report
-------------------
t/001_mysql_sharding_client.t (Wstat: 26112 Tests: 14 Failed: 1)
  Failed test:  14
  Non-zero exit status: 102
  Parse errors: No plan found in TAP output
Files=1, Tests=14, 22 wallclock secs ( 0.02 usr  0.00 sys +  7.97 cusr  6.61 csys = 14.60 CPU)
Result: FAIL