*: change max_allowed_packet to 64MB

[csuzhangxc]

What problem does this PR solve?

the default value of max_allowed_packet in MySQL server and go-sql-driver/mysql driver is 4MB, but in TiDB server it's 64MB.
ref:

• https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_max_allowed_packet
• https://github.com/go-sql-driver/mysql#maxallowedpacket

What is changed and how it works?

• change the default value of max_allowed_packet to 64MB
  • in fact, in the clinet this can be 1GB, ref https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_max_allowed_packet
• add a max-allowed-packet config item for DB config
• change charset from utf8 to utf8mb4

we can refactor DB operation to a pkg and add test cases for config later.

Check List

Tests

• Manual test (add detailed scripts or steps below)
  • config with/without max_allowed_packet in DM-worker's config and task's config
  • hack the code to see the DSN

Side effects

• Increased code complexity

[IANTHEREAL]

LGTM

[csuzhangxc]

/run-all-tests