Replace TPIE sorting by partioning and merging fsa's

[hendrikmuhs]

#180 made my think.

For some reason my vision was to replace the sorting code. I realized this might not be necessary, we have everything we need.

I compared the suggestion I gave in #180 on larger data sets:

• creating a keyvi file from scratch, utilizing TPIE
• creating x small keyvi files (using the "small data compilers" which don't use TPIE sort) and run merger on it

I ran different cases, in summary the merge approach was roughly 20% slower. Note, I did not optimize anything (I used simple python scripts). My merge approach had to copy more data, an improved implementation would avoid that.

The idea is as follows

1. create an in-memory sorter
2. if the in-memory sort buffer hits the threshold, sort the data, create an fsa, persist it, free buffers
3. go to 1.
4. after all data has been processed, sort, create, persist the final chunk
5. merge the fsa's and create the final keyvi file

[narekgharibyan]

@hendrikmuhs yeah, I really liked this. And this approach was tested on production on large scale datasets, so def worth doing!