Flush dirty entries in quick_clear(), and rename to clear()

Untaint values read from file in _set_options() (see email)
	Actually, only the header values should be untainted
	Cache entries should be tainted

Implement _lock()/_unlock() in XS to make more portable
	May as well do the mmap() in private XS as well, to reduce dependencies

Make thread-safe

Factor common code out of read() and write()

Proper test suite

