blob: added hacks for elliptics EXTHDR flag

Now all writes with BLOB_DISK_CTL_EXTHDR flag set are treated specially.
This increases complexity of eblob code, but speeds up elliptics and
greatly simplifies it's backend code.

NB! All hacks are located under `if (flags & BLOB_DISK_CTL_EXTHDR)`
blocks so it should not affect other users of libeblob except elliptics.

Requested by: zbr@