update valid/broken logic to hybrid approach

[coder543]

update valid/broken logic to hybrid approach

• use PING for (optional) checkout validation
• use the is_open() check for checking back into the pool

After discussing this with some other people, it sounds like the r2d2 checkout logic is supposed to be more "active" in validating the connection than simply checking the connection state, so I propose using this hybrid approach.

@badboy do you have any thoughts on how involved the validation logic for checking out a connection from the connection pool should be? I appreciated your responsiveness on merging my previous PR.

[badboy]

Hm, the docs specifically say that the is_valid check would do a simple query and the has_broken should be fast, thus I'm fine with your changes now.

[badboy]

(the r2d2 docs I mean)