Buffer fixes

[frikilax]

:sparkles: Buffer fixes and improvements

:page_with_curl: Type of change

Please delete options that are not relevant.

Bug fix: non-breaking change which fixes an issue.

:bulb: Related Issue(s)

• Resolve #212

:black_nib: Description

Fixed

• [BUFFER] Filter crashed when source wasn't a string
• [BUFFER] Filter crashed sometimes with multiple outputs
• [BUFFER] The sum filter did not work for "zero-length" cache
• [BUFFER/UNAD] Wait for interval before launching first thread functions

Added

• [BUFFER] Expire cache entries to prevent leftovers in Redis

:dart: Test Environments

Ubuntu (18.04)

• Redis (4.0.9)

:heavy_check_mark: Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] (If other changes) I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes

• [ ] :raising_hand: I certify on my honor that all the information provided is true, and I've done all I can to deliver a high quality code