enable vert.x to limit the size of allowed requests to avoid OOM.

Signed-off-by: wangjun <wangjunbo924@gmail.com>
let ConnectionPoolTooBusyException extends VertxException instead of RuntimeException

Signed-off-by: wangjun <wangjunbo924@gmail.com>
Merge pull request #849 from wangjunbo924/junbo-2.0-pr

enable vert.x to limit the size of allowed requests to avoid OOM