This CR adds WAL to etcdraft to persist log entries, so
that an etcdraft based chain can continue from where it's
left. The index of log entry is persisted with block as
its metadata so written blocks are not replayed.

Change-Id: I7e4131541d90b256d767a9421ab6ae2a47a88bcd
Signed-off-by: Jay Guo <guojiannan1101@gmail.com>