Update dependencies that contains known vulnerabilities

[Apelsinka223]

Two dependencies arc-swap and futures-task are vulnerable to security issues.
Source:

• https://rustsec.org/advisories/RUSTSEC-2020-0091.html
• https://rustsec.org/advisories/RUSTSEC-2020-0060.html

This PR updates those dependencies to versions that fixes the issues:

• futures: 0.3.4 -> 0.3.28, including inner dependencies update;
• arc-swap: 0.4.4 -> 0.4.8

[vtm9]

great pr @Apelsinka223 , hope it will be merged soon