libhdfs++: Make log levels consistent

Most of the public C++ FileHandle/FileSystem operations have a LOG_TRACE level message about parameters passed in etc. However many methods use LOG_DEBUG and a couple use LOG_INFO.

We most likely want FS operations that happen a lot (read/open/seek/stat) to stick to LOG_DEBUG consistently and only use LOG_INFO for things like FileSystem::Connect or RpcConnection:: that don't get called often and are important enough to warrant showing up in the log. LOG_TRACE can be reserved for things happening deeper inside public methods and methods that aren't part of the public API.

Related improvements that could be brought into this to avoid opening a ton of small Jiras:
-Print the "this" pointer address in the log message to make it easier to correlate objects when there's concurrent work being done. This has been very helpful in the past but often got stripped out before patches went in. People just need be aware that operator new may eventually place an object of the same type at the same address sometime in the future.
-For objects owned by other objects, but created on the fly, include a pointer back to the parent/creator object if that pointer is already being tracked (See the nested stucts in BlockReaderImpl).