DLFS - A FileSystem API wrapper over dlog API

[sijie]

Descriptions of the changes in this PR:

• FileSystem API wrapper built over dlog API

(This is based on initial implementation from @gerritsundaram at #43)

Features supported:

• create and append files
• open files for reading
• input stream and output stream for reading and writing data
• list files
• get file status
• rename
• mkdir

Features aren't supported:

• truncate
• currently there is no clear distinguish between file and dir
• only support delete recursive

(This change includes small changes for #224 #225 #226 ).

[sijie]

@jiazhai can you review this again?

[sijie]

addressed the comments.