blob: use new writev()-like libeblob APIs

This greatly simplifies backend code by moving all work behind eblob's
BLOB_DISK_CTL_EXTHDR flag.
blob: use eblob_read_return() instead of eblob_read_nocsum()
blob: fixed formatting
blob: added shortcut 'b' to 'c->eblob'
leveldb: fixed write of timestamp/user_flags
leveldb: removed leveldb backend

Requested by: zbr@
recovery: python 2.6 compat
recovery: do not emit misleading error message
recovery: check that recovery address was speficied
recovery: check that atleast one route was returned
XXX: tests: cpp