Added threadpool to WebsiteSource

[hoxbro]

I just thought it made a lot of sense to check the websites asynchronously.

I have set a 10 seconds timeout, which could be removed again or given as a parameter.

[codecov-commenter]

Codecov Report

Merging #344 (cc2afdc) into master (a503238) will decrease coverage by 0.02%. The diff coverage is 23.52%.

@@            Coverage Diff             @@
##           master     #344      +/-   ##
==========================================
- Coverage   67.22%   67.19%   -0.03%     
==========================================
  Files          71       71              
  Lines        7340     7349       +9     
==========================================
+ Hits         4934     4938       +4     
- Misses       2406     2411       +5     

Impacted Files	Coverage Δ
lumen/sources/base.py	62.93% <23.52%> (-0.30%)	:arrow_down:

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more