Fix memory leaks caused by multiple start calls

[art5255]

if start called multiple times should clear previous interval to avoid memory leak

[coveralls]

Coverage decreased (-1.04%) to 98.958% when pulling 264e698 on art5255:master into 6632b49 on mironov:master.

[mironov]

@art5255 Thank you for the PR. Any idea of how several sequential calls of start() could happen? I wonder if a test can be written for the case.

[mironov]

I think I fixed a possible case when start() could be called twice resulting in leaking progressInterval in 832727c
I'm going to close this PR for now.

Feel free to rebase and reopen it if the issue still persists in >= v4.5.0