Add a hook into the connection initialization phase

[nikita-volkov]

Any opinions or wishes for changes on this?

@steve-chavez @robx @domenkozar

[steve-chavez]

@nikita-volkov What's the motivation for this PR?

[nikita-volkov]

To provide a way for the user to issue some commands on every established connection prior to it becoming available for execution of statements thru the pool. E.g., to configure connections by issuing a bunch of SET commands.

[steve-chavez]

Ah nice, it enables session variables then. If the connection dies, will the session variable survive?

[nikita-volkov]

I'm not sure what you imply by connection dying and variable surviving.

The same session will be reexecuted each time a connection gets established by the pool for whatever reasons. That means that it'll be safe to assume that every connection used by the pool will be running having that session executed.