Provision to store metadata outside gzinga file.

[brahul]

There can be an situation ,where the user might be interested in storing the metadata (long,offset) outside of the gzinga file and only create markers in the file for subsequent random access.
This would also enable the user to associate any kind of metadata with a gzinga file as the index can now be stored outside of the gzinga file and it doesn't have to be <Long,Long>.