Check memory usage only if jedi is used and reduce frequency of these checks

[DonJayamanne]

Fixes #1277

Fundamental changes:

• Use settimeout instead of setinterval (the requests were getting piled up with setinterval).
• I've reduced the frequency of checks from 2 seconds to 15 seconds.
• Check memory usage only if jedi is used>

This pull request:

• Has a title summarizes what is changing
• Includes a news entry file (remember to thank yourself!)
• [n/a] Has unit tests & code coverage is not adversely affected (within reason)
• [n/a] Works on all actively maintained versions of Python (e.g. Python 2.7 & the latest Python 3 release)
• [n/a] Works on Windows 10, macOS, and Linux (e.g. considered file system case-sensitivity)