Improve certstorage example

[tetafro]

I've added more info and one more example to CertStore example.

I actually just looked through these discussions and copied useful information, so all credits to people from there:

• Pluggable certificate storage (following on from #284) #332
• Pluggable certificate storage (following on from #284) #330
• Add possibility to install a cert store #284

[ErikPelli]

Having a mutx for every single host looks like an anti-pattern to me.
This will also cause memory leaks if there are a lot of different hosts, since the older one are never cleaned uo.

[tetafro]

I don't really remember my thinking from 3 years ago, but looks like it's just the most straight-forward example I could come up with.
This definitely can be improved in real life, using some caching with eviction policy instead of a map.

[ErikPelli]

Uhm ok, do you think it should still be merged or the current simple example is enough as a simple example for someone that never used this library? (Do you still use it?)

[tetafro]

Wait, I just noticed that you did pretty much the same in the current example. Why are we discussing this PR then? Let's just close it.