MB-11133: dont use second as part of directory

When using timestamp as backup directory structure, second unit
is too sensitive for multi-bucket/multi-node case.

Change-Id: I835b55c8ec428f11c32432d8b840ceea4e4c5d53
Reviewed-on: http://review.couchbase.org/37301
Reviewed-by: Bin Cui <bin.cui@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>