Even if random chars are all 0, at least the first one isn't
so we know we can't return INVALID_UUID.

Admin aborts by sending an error packet for each request.
Without this, the neoctl would wait forever.

This fixes admin not always forwarding error packets from master to neoctl.

This is required, for example, for simultaneous reciprocal replication
between 2 storages.

Also, make it call _parse to simplify subclasses.

For example, with test2Clusters:

Traceback (most recent call last):
  File "neo/master/app.py", line 111, in run
    self._run()
  File "neo/master/app.py", line 131, in _run
    self.playPrimaryRole()
  File "neo/master/app.py", line 318, in playPrimaryRole
    self.provideService()
  File "neo/master/app.py", line 269, in provideService
    em.poll(1)
  File "neo/lib/event.py", line 90, in poll
    self._poll(timeout=timeout)
  File "neo/tests/threaded/__init__.py", line 150, in _poll
    Serialized.tic(self._lock)
  File "neo/tests/threaded/__init__.py", line 102, in tic
    cls.release(lock); cls.acquire(lock)
  File "neo/tests/threaded/__init__.py", line 88, in acquire
    elif threading.currentThread() in cls.pending:
TypeError: argument of type 'int' is not iterable

This commit is similar to 900d4908.

This fixes commit d90c5b83
("Allow NEO to store empty values").

This optimizes storage layout on disk, because more recent entries are often
being accessed more.

This will also simplify implementation of incremental backups.

Storage API is changed so that backends are not forced to use the checksum to
index data.

Values of '_oid_dict' attribute are never empty.

- Fix deadlock when client is in slave mode whereas a DB was open.
- Fix regression in commit b3b5175f
  ("tests: make it possible to run several threaded clusters at the same time").