saving code refactored a bit, added a function returning the number of bytes an object will use on disk

Fixed a minor bug in GETSET, now the SET part is not performed if the GET fails because the key does not contain a string value

SHUTDOWN now does the right thing when append only is on, that is, fsync instead to save the snapshot.

LZF compression re-enabled by default, but with INIT_HTAB set to 0 to avoid the very costly memset initialization. Note that with this option set valgrind will output some false positive about lzf_c.c