Snapshotting is taking more than 10 mins.

[tejal29]

Actual behavior
Snapshotting FS should for a Node js project with npm install command should be faster than current.

Expected behavior
Looks like TakeSnapshotFS is a bottleneck for projects with huge number of files.
e.g .for a project with file count 800K, the TakeSnapshotFS takes almost 15-20 mins.

After some investigation, i found out we loop over files multiple times. In this PR, we are reducing the number of times we loop over the files.

[sameersbn]

I am facing this issue as well. this step takes the most number of cycles in my builds. optimizing this will greatly reduce the build time of ALL my images.