udpa: clarify URI fragment encoding of directives. (#30)

After spending some time looking at RFC3986 and chatting with Louis, it
seems we can support multiple directives in the fragment, but we need to
be very precise on how they are percent encoded.

This patch proposes that we move from having multiple fragments (which
are not allowed in RFC compliant URIs) to a single fragment with
comma-separated percent encoded directives. Comma was selected as it is
an available character in RFC3986 sub-delims, intuitively indicates
separation, and is less likely to appear in URIs (forcing percent
encoding) than other alternatives, e.g. '&' which would appear in most
URIs with context parameters.

Percent encoded directives in URIs should remain fairly readable, for
example udpa://foo#entry=something,alt=udpa://bar/baz&x=y requires no
percent encoding.

Also renamed schema to scheme to match RFC3986.

Signed-off-by: Harvey Tuch <htuch@google.com>