Index generation is slower than previous implementation

[ajnavarro]

Log indexing a table with the actual version:

DEBU[0461] finished pilosa indexing                      duration=5m26.700323786s id=blobs_idx mapping=508129748 pilosa=384190894 rows=55764

Log with previous implementation, the same table:

DEBU[0022] finished pilosa indexing                      duration=5.323093051s id=blobs_idx mapping=429.571981ms pilosa=268.452932ms rows=55796

5 mins vs 5 secs.

[kuba--]

[kuba--]

After creating mapping per partition we get better performance:

[kuba--]

you can see that bbolt.Commit dropped from 3.39s to 0.06s

[kuba--]

The fix will break backward compatibility, because we'll require mapping file per partition.

[erizocosmico]

It's a 0.x, so I guess it's fine to break compat