ITS#9555 Introduce a default operations timeout for back-asyncmeta

This patch changes the default operations timeout for asyncmeta to 2 seconds, to prevent no timeout being set due to configuration error or lack of familiarity with asyncmeta. A timeout of 0 can still be configured, if no timeout is the desired outcome. Using asyncmeta without operation timeouts is potentially unsafe - if for some reason a target server does not return a response, the operation will stay in the queue until the connection is reset due to some network error (idle-timeout is not honored if there are pending operations). In the case of many such operations, the queues can get filled up and asyncmeta will become unable to process new operations for some time.

Therefore, a setting of 0 should be used carefully, only in environments where a response is guaranteed.