libhdfs++: Public API headers should not depend on internal implementation

Public headers need to do some combination of the following: stop including parts of the implementation, forward declare bits of the implementation where absolutely needed, or pull the implementation into include/hdfspp if it's inseparable.

Example:
If you want to use the C++ API and only stick include/hdfspp in the include path you'll get an error when you include include/hdfspp/options.h because that goes and includes common/uri.h.

Related to the work described in HDFS-10787.