Extended format management in eblob, stats and tests improvements

* Replaced general purpose `BLOB_DISK_CTL_USR1` flag with special `BLOB_DISK_CTL_EXTHDR` which changes eblob behavior in various ways so that metadata management in elliptics can be implemented efficiently;
* Fixed `writev()` behaviour for `iovcnt` > 1;
* Tests improvements: added `writev()` tests, improved append and three stage write;
* Proper cleanup of bases which are removed due to absence of entries;
* Many statistics fixes;
* Comment and logging improvements;
* Various static analyzer/pedantic fixes.

@zbr: New release and major version bump are needed.