Sequential log

I have done major modifications in both the logging_vnode layer and the replication_vnode layer. From outside, everything works exactly the same (so the API remains the same). The main change is:

The log now stores all the operations sequentially no matter to which Key they belong to. Thus, the inserting order is preserved and simplifies some things for other vnodes such as the interDC vnode and Downstream vnode of other branches.

This new approach matches the decisions taken in LLN during our June informal meeting. I have passed all the tests : floppystore, gloppy_log, floppy_log_handoff and append_list_test