made FileManager store File and not open it each time

[DobbiKov]

Overview

As mentioned in the #9 the logic of writing to a file has been changed. Before, each time we wrote a log to file, we opened the file, wrote a log, closed the file.

This pull request changes this logic to storing the file in the memory (so it opens once), and closes only on rotation or when the application closes.

Performance comparison

Printing 10000 logs and writing to a file

Before:

Time elapsed: 239.863125ms

After:

Time elapsed: 69.116042ms

Printing 1 million logs and writing to a file

Before:

Time elapsed: 21.669482208s

After:

Time elapsed: 5.41291075s