HTTP/2: limit the number of idle state switches. An attack that continuously switches HTTP/2 connection between idle and active states can result in excessive CPU usage. This is because when a connection switches to the idle state, all of its memory pool caches are freed. This change limits the maximum allowed number of idle state switches to 10 * http2_max_requests (i.e., 10000 by default). This limits possible CPU usage in one connection, and also imposes a limit on the maximum lifetime of a connection. Initially reported by Gal Goldshtein from F5 Networks.