This patch allows using WCFS with a NEO or ZEO storage which is
reachable by a URL which contains an ipv6 host.

Without this patch the following example doesn't work:

>>> from wendelin.lib.zodb import dbopen
>>> root = dbopen("neo://cluster-name@[::1]:2051")
>>> # "abc" points to a ZBigArray
>>> root["abc"][0]

It doesn't work because the parser missed adding square brackets around
ipv6 hosts, due to which unparsing the resulting URL resulted in a wrong
interpretation where a port starts.

This patch furthermore amends 'test_zstor_2zurl' to test ZEO and NEO
storages with ipv6 hosts.

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/reviewed-by @kirr
/reviewed-on nexedi/wendelin.core!13