MB-11133: Use one timestamp per session

Under one session, each individual bucket/node pair should share
same timestamp for backup directory.

Change-Id: I847d5f6c37440903c1672f7b42219f45c506acd3
Reviewed-on: http://review.couchbase.org/37326
Reviewed-by: Bin Cui <bin.cui@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>