In the old source code we already filtered NEO URI by dropping
credentials, but we applied this filter to any URI, not only the NEO
one. This patch adds a mechanism to apply various filter according to
the specific storage type. Starting with this new patch,
'zurl_normalize_main' also refuses to normalize an URI with an unknown
scheme.

/reviewed-by @kirr
/reviewed-on nexedi/wendelin.core!17