New eblob 0.21.0 API, metadata fixes, leveldb backend removal and recovery improvements.

Elliptics:
* Use eblob's new `BLOB_DISK_CTL_EXTHDR` flag and `writev()`-like API. This moves most of metadata complexity from elliptics backend to `libeblob`  
**NB!** This is very complex change - needs excessive testing;
* Removed `leveldb` backend along with `snappy` dependency;
* Fixed read with offset/size specified;
* Cleaned up leftovers from compression removal;
* Added tests for reads/writes with offset and size specified;

Recovery:
* Simplified output for most counters;
* Fixed python 2.6 compatibility;
* Usability improvements;
* Added computation of hash ring % in merge mode;

@zbr: New eblob release `0.21.0` is needed for this to compile.