Pyro4

- fixed Python 3.6 compatibility issue in name server when using sqlite storage ("cannot VACUUM from within a transaction")
- fixed Python 3.6 ResourceWarning in unit test
- Python 3.6 added to travis CI build
- fixed possible crash on Python 2.x when shutting down a daemon from within a Pyro server object itself (because it tried to join its own thread)
- sensible error is raised again in client when threadpool server can't accept any more new connections (regression since 4.50)
- daemon has new `resetMetadataCache` method to be used when the set of exposed members of your Pyro class changes during runtime
- better testcases for properly handling handshake error reasons

- added PYROMETA magic URI protocol, to look up an object with the given metadata tags (yellow-page lookup rather than by name)
Example: `Pyro4.Proxy("PYROMETA:metatag1,metatag2")`
- added distributed-computing3 example to show simple work load distribution using PYROMETA object discovery
- fixed unlikely but possible crash in logging statement when client disconnects from multiplex server

- new ITER_STREAM_LINGER config item to keep streams alive for a given period after proxy disconnect (defaults to 30 sec.)
- new NS_AUTOCLEAN config item to set a recurring period in seconds where the Name server checks its registrations.
It will then auto cleanup registrations after a short while if they're no longer available. (defaults to 0.0 - disabled).
- Future can now be given a delay before it is evaluated
- Future can now be cancelled (if it hasn't been evalued yet)

- added iterator item streaming support. It is now possible to return iterators from a remote
call or even call a remote generator function, and iterate over it in the client.
Items will be retrieved on demand from the server.
- new ITER_STREAMING config item to disable or enable streaming support in the server (default=enabled)
- new ITER_STREAM_LIFETIME config item to be able to set a maximum lifetime for item streams (default=no limit)
- the iter streaming is supported for Java and .NET in Pyrolite 4.14 as well
- new simplified stockquotes example using generators instead of callbacks
- changed daemon shutdown mechanism again to not use separate thread anymore, fixes thread leak
- serpent library dependency updated to 1.15

- The threaded socket server now adapts the number of threads dynamically depending on connection count.
This resolves the problem where your clients freeze because the server ran out of free connections
When all threads are busy, new connections will fail with an exception.
- THREADPOOL_SIZE_MIN config item added to specify the min number of threads (defaults to 4)
- THREADPOOL_SIZE increased to 40 (was 16, and no longer allocates all these threads upfront)
- THREADPOOL_ALLOW_QUEUE config item removed, it is no longer relevant
- made the repr strings use semicolons instead of comma as separator to avoid confusion when printed in lists
- added per proxy serializer override by setting proxy._pyroSerializer
- added distributed-mandelbrot example that shows ascii animation and picture of the mandelbrot fractal set
- fixed timeout when locating name server on 127.0.1.1 on systems that don't use that address (osx)
- fixed ResourceWarning in socketutil.createSocket; socket that could not be connected is properly closed now

- _Backwards incompatible change:_ As announced in the previous version, the instance_mode and instance_creator
parameters have now been removed from the expose decorator. Use behavior to specify them instead on your classes.
- The default instance mode when using expose on the class and not using behavior, is now also 'session'
(was 'single'). Note that when you used behavior with its default argument or only expose on methods,
the instance mode of the class already was 'session'.
If your code really requires the pyro object to be a singleton, add an explicit
behavior(instance_mode="single") to that class. You can already start doing this while still using Pyro 4.46 and
then upgrade the library once you've converted everything.
- Name server lookup now also considers 127.0.1.1 when trying to find a name server on localhost.
This is convenient on some systems (Debian Linux) where 127.0.1.1 is often the address assigned
to the local system via the hosts file.
- fixed multiplex server shutdown sometimes hanging
- fixed crash that sometimes occurred in daemon shutdown
- fixed crash that sometimes occurred when releasing and reconnecting the proxy from different threads