MB-11934: Fix warmup for windows

During warmup in windows, the platform library appends a backslash instead
of a forward slash causing a database file name parsing to break.

Change-Id: I4b3d2296c83dcdf4f94698532a2115dedc841fdb
Reviewed-on: http://review.couchbase.org/40673
Tested-by: Sriram Ganesan <sriram@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>