rbh-config: new option 'reset_acct' to rebuild accounting info.

"rbh-config reset_acct" drop the ACCT_STAT table,
causing robinhood to rebuild it on startup.

Change-Id: I3042f88575e5612a8d91adb3ad749e8bb7ebca14
backup: rbhext_tool timeout must be < robinhood's copy_timeout

Change-Id: I02495836a1e300c0ead8c1636bdf755c97bc43e8
rbh cmd: doesn't warn if someone uses both changelog and scanning.

Change-Id: I369fb35ad0f5e21ff15a6cc6339f5b37cd69ef68
migration policy: fix invalid handling of lstat error.

Invalid handling of lstat error resulted in triggering copy
for files that no longer existed.

Change-Id: I8231a4993ac93d5471c06e96256ec908fcb9df33