This patch adds `demo:` URI scheme to create DemoStorage from an URI.

While several existing resolvers already handle ?demostorage argument to
wrap itself with DemoStorage and in-RAM MappingStorage for changes, this
approach has the following drawbacks:

- every resolver must do it
- it is not possible to create DemoStorage with non-MappingStorage for changes.

My particular motivation here is Wendelin.core 2: it spawn WCFS
filesystem server to serve array data from ZODB storage, and passes
storage URL to spawned wcfs process, so that wcfs could connect and
retrieve data from the same ZODB storage that client process is
using. When original ERP5 client is using DemoStorage, both `base` and
`changes` must be persisted because if changes would be in-RAM
MappingStorage, WCFS could not access that data because it runs as a
separate process.

To build a DemoStorage URI we follow XRI Cross-references approach to
embed URIs for base and changes into combining demo URI:

    demo:(base_uri)/(δ_uri)

https://en.wikipedia.org/wiki/Extensible_Resource_Identifier provides
some related details and examples.

I choose fragments as the place for ZODB.DB arguments:

    demo:(base_uri)/(δ_uri)#dbkw...

The reason fragments - instead of parameters - are used, is because
DB arguments are _local_. Even with different DB arguments the URI
refers to the same storage, and for wendelin.core 2 it is important to
be able to determine whether two client processes actually use the same
underlying ZODB storage, even if those two clients use different local
parameters, like connection_pool_size and similar. Keeping such local
arguments in fragments makes it easy to determine the underlying URI of
the storage - by dropping fragments. It also follows logic from RFC 3986,
which says that "fragment ... may be ... some view on representations of
the primary resource": https://tools.ietf.org/html/rfc3986#section-3.5

Thanks beforehand,
Kirill

/cc @tseaver, @azmeuk, @jimfulton

P.S.

It was already suggested several times to add DemoStorageURIResolver and
deprecate ?demostorage in existing URI resolvers:

https://github.com/Pylons/zodburi/pull/25#issuecomment-485506959
https://github.com/Pylons/zodburi/pull/25#issuecomment-511480572
https://github.com/Pylons/zodburi/pull/25#issuecomment-622931793