Rebase 8.0.20 code

[yoku0825]

Why 8.0.20 is to avoid this Issue,

  8.0.20 - Not affected
  8.0 21 - Not tested
  8.0.22 - Affected
  8.0.23 - Affected

• MySQL Bugs: #101406: MySQL Router "resource temporarily unavailable" in log file - and client hanging

[yoku0825]

My old implementations was mikasarouter-8 linked MySQL 5.7's libmysqlclient.so statically at build time (maybe)

MySQL Router 8.0.13 and later are merged into MySQL Server's code tree.
I can build MySQL Router 8.0.20 + fabric+cache plugin, but the binary is linked MySQL 8.0's libmysqlclient.so.

mikasafabric daemon used old MySQL Protocol and MySQL 8.0's libmysqlclient.so can not connect it.
( You can verify this behavior by trying to connect from MySQL 8.0's mysql command line client to mikasafabric daemon ).

Workaround is there, password exchange is caused connecting error, then all user's password set to empty-string can avoid this connectivity error.

Is it pragmatic?