Check for supported scheme in `create_handler` about pfio

[belldandyxtq]

It seems other APIs do not have this issue as they accept the UNIX-like path as well as the scheme, and 'boo' will be treated as a UNIX-like path.

[kuenishi]

Reopen as #88 is just a workaround. Discussion should be continued here.

[belldandyxtq]

The argument of create_handler(scheme) is kind of misleading, as other functions and methods take URI as arguments. That said, more intuitive improvement would be to let it accept full or partial URI and make .root member configurable and effective.

I think this was the actual behavior before #86 with #87 fix.

Also we have to support choosing HDFS-cluster at each handler, which is currently not available.

This would involves two ToDos:

1. Switching between HDFS-cluster would involve switching the connection. Or we can maintains connections to all the available clusters in hdfs.xml.
2. Differentiating the cluster-name from directories

[belldandyxtq]

@kuenishi What do you think about this

[kuenishi]

My suggestion is to change argument of create_handler from just a scheme to full URI that indicates some directory in some cluster, or in local filesystem. This change involves a lot of code change like you suggested, and maybe to support creating handlers like chdir-like style. Not only about full URI but relative URI or pathnames are to be in discussion scope.