Spurious innodb_io_capacity-sized flush list flushes on a loaded server

The furious flushing, implemented by skipping page cleaner thread sleeps, in combination with short-duration LRU and flush list flushes, means that server activity might be not updated by the next page cleaner thread iteration.

This has an effect of spuriously issuing innodb_io_capacity-sized flush list flush request instead of flushing as loaded server is supposed to, which in turn causes performance instabilities.

The fix is to maintain server activity accounting based on 1 second granularity. That is, an active server may only be considered inactive if no server activity bump in the last 1 second, as opposed to last cleaner thread iteration. An inactive server may be considered active immediately after the server activity bump.