Extend configuration with a timeout of connection being idle

[nikita-volkov]

Discussion of this is initiated in #28.

[robx]

Just to note that with #28, this should be straightforward to implement now (but it's probably also no longer that pressing):

• add connUseTimeNSec next to connCreationTimeNSec, poolMaxIdleTime next to poolMaxLifetime
• add a check involving these to isAlive
• update connUseTimeNSec in use's onConn

(and a test :))