Bug 3441: Part 1: Minimize cache size corruption by malformed swap.state.

Bug 3441: Part 1: Minimize cache size corruption by malformed swap.state.

If swap.state gets corrupted, a single entry with bogus size value will screw
up Squid idea of the current cache size. A newly added StoreSwapLogData sane()
method attempts to minimize the chance of corruption by ignoring log entries
with obviously bogus values.

However, without expensive size checks (-S or "Does the log entry matches the
actual cache file size?"), it is not possible to reliably detect all bogus log
entries.

If Squid gets a wrong idea of the current cache size, it may either cache too
much (and possibly run out of space) OR delete everything.