remove closed connections from pool to avoid potential memery leak.
Following is the detail of the issue for those who are interested in itJ

The issue happens when adapters respond a non-success http code.(Perhaps not all non-success code , but I’m sure 404 will 100% re-produce this issue) . In this case an closed connection will be referenced by the connection pool which leads to: 1) the connection pool holds more and more closed connections. It’s a memory leak. The memory keeps increasing.   2) every time a new request try to select a connection, it will iterate all connections. As the pool size increases, more and more CPU time are wasted.   3) By default a closed connection won’t be used. However, under high load, it’s easily the count of in-use connections reaches the threshold: 10 by default. Then vert.x will try to assign an closed connection to request. And since its’ closed the request will try to pick up another connecton(The first one can be released now).  Probably vertx assign another closed connection. It’s an infinite loop which cause “StackOverFlowError”.
Merge pull request #4 from wangjunbo924/master

remove closed connections from pool to avoid potential memory leak.