Fix conflict handling after a successful store to a node being disconnected for having missed a transaction

- fail sooner in case of unresolvable conflict
- avoid OOM when there are many conflicts

Now that we do inequality comparisons between timestamps, the master must
use a monotonic clock, to avoid issues when the clock is turned back.
Before, the probability that time.time() returned again the same value was
probably negligible.

This optimizes the normal case, and handlers can now take specific action
when requests are cancelled because a connection is closed.

It was only used by the now removed HasLock.

It was disabled long time ago and NEO has evolved in such a way that the new
implementation will be completely different.

It's dead code, because 1 year after it was introduced, something else was
implemented to detect deadlocks immediately.

Anyway, it would be an unacceptable way to detect them.

It was failing when some .py files were not compiled.

100 is too small for tests like testBasicStore (MySQL) and testDelayedStore.

When receiving 1 byte, benchmarking shows no visible difference with values
between 4096 and 65536 for the buffer size. With higher values, it becomes
significantly slower.

On the other side, a 64k buffer is faster with bigger packets.
Time to run testBasicStore with MySQL:

            4096        65536
    real    0m51.115s   0m21.592s
    user    0m41.857s   0m13.540s
    sys     0m8.700s    0m2.687s

It started to fail with commit fd007f5d.

It should have been removed with commit cf32e594
(see also c277ed20 and related commits).

Instances of NEOCluster were not deleted as soon as the only referrers were
weak proxies (at least that's what a quick check with the 'gc' module showed
at the beginning of tearDown). In some cases, __del__ was called while the next
test was logging a message, which led to deadlocks.

Without those proxies, it may be reliable, but only on CPython. See
  http://doc.pypy.org/en/latest/cpython_differences.html#differences-related-to-garbage-collection-strategies

Relying on __del__ to close a cluster was wrong. NEOCluster is now a context
manager that does it explicitly at exit, in addition to automatically stop it.
The NEOCluster.stop method combines the previous stop/__del__/reset methods.

A new 'with_cluster' decorator is also added to avoid excessive indentation
in tests. Unindentation of existing tests will be done later.