Add rate limiter for insertions and deletions

[cgzones]

Add options to rate limit insertion and deletions. Disabled by default.

[matze]

Can't we use this tower layer instead of pulling in yet another dependency?

[cgzones]

Can't we use this tower layer instead of pulling in yet another dependency?

One could, but I tried to achieve something different: Such a layer will rate-limit all requests, including viewing existing pastes. This can also being achieved via a DDOS in a proxy webserver, used for TLS encryption (nginx/apache).
I tried to explicitly limit the addition and deletion of pastes to mitigate spamming and bruteforcing the deletion of a specific paste a bit.

[matze]

I'll slate this and the other open PRs for the release after the next one. A lot of good and non-controversial stuff has piled up already and deserves to be properly released.

[matze]

I was thinking a bit about the usefulness. First of all: I hope no one is running this pastebin facing the internet and I don't think that we should add complexity that could be mitigated differently for internal networks. Second: isn't this penalizing everyone instead of the caller?