Restore BDB block index wrapper and enable it by default.
Make sure we always have a node to do IBD from

This introduces the concept of the 'sync node', which is the one we
asked for missing blocks. In case the sync node goes away, a new one
will be selected.
Add bytessent, bytesrecv and syncnode to getpeerinfo
Merge pull request #26 from CryptoManiac/master

Restore BDB index wrapper, add sync node concept