Update URI parser to use SBuf parsing APIs (#275)

Update URI parser to use SBuf parsing APIs (#275)

Initial replacement of URI/URL parse method internals with
SBuf and Tokenizer based parse.

For now this parsing only handles the scheme section of
URL. With this we add the missing check for alpha character
as first in the scheme name for unknown schemes and
prohibit URL without any scheme (previously accepted).

Also polishes the documentation, URN and asterisk-form
URI parsing.

Also, adds validation of URN NID portion characters to
ensure valid authority host names are generated for
THTTP lookup URLs.