Implement simplified faucet

[jkrvivian]

Description of change

Type of change

• Breaking change

Change checklist

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

[piotrm50]

We should use client/wallet package to manage funds and addresses instead of having to manage it in the faucet plugin. It'd make the code more simple and readable.
If we configure the wallet to reuse the same address, then it will always read the correct state from the tangle.
It also would be nice to only handle requests that are not older than X minutes, because otherwise in case of faucet node restart, it would try to fullfill all the requests again.

[piotrm50]

Remove the faucet request window check, fix the tests and I think it's good to go :)

[jonastheis]

It looks already pretty good and makes everything so much simpler. But I think some things can still be improved