[9.x] Support connection to MySql with minimal interactions

[coppolafab]

Reducing configuration statements issued to MySql Server on connect, like 'use db' or 'set names', helps taking full advantage of a connection proxy, minimizing chances of connection pinning [1].
Such configuration statements can also be executed once per connection, via proxy's initialization query, instead of once per Request lifecycle.

A support for a minimal interaction upon connection can also be useful in some conditions (es: tests, low resources).

Actually, on connect, MySqlConnector executes:

• Connect (via new PDO)
• use db query (if set $database)
• SET SESSION TRANSACTION ISOLATION (if set $isolation_level, as prepared statement)
• set names (if set $charset, as prepared statement)
• set time_zone (if set $timezone, as prepared statement)
• set session sql_mode (if set $modes or $strict, as prepared statement)

This PR:

• removes the 'use db' query, already specified into the DSN
• adds the charset (if set) into the DSN, with the same effect of 'set names' [2],
  also ensuring proper PDO::quote functionality, not affected by 'set names' statement [3].
  'set names' will be executed necessarily if a collation is set but not as prepared statement, useful when not using the default one, but unsetting it allows skipping this step.
• adapts existing tests to reflect changes

Setting these database options to NULL (isolation_level, collation, timezone, modes and strict) allows having a minimal interaction on a connect, like:

2020-07-24T12:51:18.507740Z	    2 Connect	db_user@127.0.0.1 on db_schema using TCP/IP
2020-07-24T12:51:18.509693Z	    2 Query	select true

[1] - https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/rds-proxy-managing.html#rds-proxy-pinning
[2] - https://dev.mysql.com/doc/refman/8.0/en/charset-connection.html
[3] - https://www.php.net/manual/en/mysqlinfo.concepts.charset.php

[taylorotwell]

Going to hold off on this for now.