keylock: make KeyLock generic and improve documentation

[jo3-l]

This PR improves the type safety of the common/keylock package by making KeyLock generic over the key type (previously interface{}), which also slightly improves performance*. For context, I was looking over common/keylock to see whether we could replace the muted user lock in https://github.com/botlabs-gg/yagpdb/blob/master/moderation/mutelock.go with a KeyLock, given the two pieces of code implement largely the same thing. (It turns out we can, but I'll defer this change to a later PR once the sqlboiler changes to the moderation plugin are complete.)

*benchstat results:

goos: linux
goarch: amd64
pkg: github.com/botlabs-gg/yagpdb/v2/common/keylock
cpu: 11th Gen Intel(R) Core(TM) i9-11900T @ 1.50GHz
           │   old.txt    │               new.txt               │
           │    sec/op    │   sec/op     vs base                │
KeyLock-16   140.9n ± 10%   115.2n ± 1%  -18.23% (p=0.000 n=10

(Though the benchmark is not at all representative of a typical use scenario, it does prove that the map access performance has improved as a result of no longer using interface{} keys. In any case, the main goal of this PR is type safety; better performance is a happy consequence.)