When a snapshot is taken, stale etcdraft WAL files should be
purged to free disk space, as well as old snapshot files.

However, we still keep several snapshot files around, in case
the latest file is corrupted, etcdraft will automatically load
an older one, until there's none left.

Change-Id: I2b8168dbc0c3e5bd56a081c104dd7dc9defbcd92
Signed-off-by: Jay Guo <guojiannan1101@gmail.com>