Added max-swap-rate=swaps/sec option to Rock cache_dir.

The option limits the rate of Rock disk access to smooth out OS disk commit
activity and to avoid blocking Rock diskers (or even other processes) on I/O.
Should be used when swap demand exceeds disk performance limits but the
underlying file system does not slow down incoming I/Os until the situation
gets out of control.

Warn if Rock disker delays are significant.