Add a generic mechanism to implement timeouts in bufferevents. Paired and asynchronous bufferevents didn't do timeouts, and filtering bufferevents gave them funny semantics. Now they all should all work in a way consistent with what socket bufferevents do now: a [read/write] timeout triggers if [reading/writing] is enabled, and if the timeout is set, and the right amount of time passes without any data getting [added to the input buffer/drained from the output buffer]. svn:r1314