gitlab-rails: Ruby-ize all dependency checks

Move to rails-dependencies script, using modules and threading, replacing looping process restarts. This serves as an exploratory look into performing as much of the dependencies checks & looping directly within Ruby.

I'm not seeing a massive performance gain over our previous changes in !433 (merged). Letting long-runs of while true ; do time /scripts/wait-for-deps >/dev/null ; done run, I am seeing an average of around 2.9 seconds with these changes. Observing the same after !433 (merged), I am seeing around 3.8 seconds, with variances between 3.56 and 4.05. It's one less second, but it could add up.

Closes gitlab-org/charts/gitlab#2165 (closed)

!433 (merged)	Seconds	!478 (merged)	Seconds	Difference
real	3.49768	real	2.91228	0.58540
user	2.17132	user	1.67872	0.49260
sys	1.27945	sys	1.18916	0.09029