hash counter rebalance API
Rebalance support:
- job and lockage semantics
- live block relocation
- block hold flag
- file relocation (via snapshot + copy + removal)
- rebalanced flag and cleanup
introduce 'node is gone' logic
node removed from the cluster withh return 410 - Gone
- http 410 is marked retriable in the client
- http 410 is equivalent to 200 in jobs that might target removed nodes
syncronize end of rebalance
sxsetup: don't call --mod if the cluster is being rebalanced (work-around for bb#660)
Also call --info right before --mod to close a possible race
Introduce single block download fallback to multi_download.

single_download() function downloads blocks sequentially in case of
multi_download() fails.
Fix fast remote to remote copy (bb#702)