[Experimental] Extended data format for elliptics and native iterators

This pull request adds highly experimental features that will be core of new recovery mechanism:
 * Extended data format that stores elliptics metadata in fixed header that prepended to data itself.
 * Iterator cmd that streams data/metadata for particular key/timestamp range over network or writes it to file.

This changes on-disk data format for all new records! It also has enormous footprint and commit volume. Use with caution. Needs extensive testing.

Known issues:
 * Newly added features like leveldb data append/offset-rewrite are not implemented. Coming soon.
 * File backend is not supported for now. 
 * No iterator state management (pause/continue/cancel)
 * Costly malloc + memcpy on header prepend.
 * Tough timestamp is written it's never filled anywhere so right now it's effectively noop (fixed trivially).