Fix command delays

[DocRevive]

As used in https://github.com/Dragory/modmailbot, each command prefilter may check a channel ID against an entire database of threads. As a standalone operation, this has an unnoticeable effect, but when there are over forty commands and nearly one hundred thousand threads, a consistent delay is present on every Modmail command run in the inbox server.

Recently, we pinpointed the source of the delay to this module. Several months ago, the support server advised us to contact Discord for selectively limiting the bot's requests to one particular server.

This change drastically improves the performance (1300ms to <1ms) of findMatchingCommand in large-scale workloads by putting the cheaper and more specific tryMatchingCommand check before the prefilters.

[DocRevive]

I realized that by switching the order this way, the error-handling block for tryMatchingCommand in findMatchingCommand would cause a response to incorrect command uses regardless of the prefilters. Now, tryMatchingCommand has been split into two parts. The first is used to check the name and prefix before the prefilters, and the second is used to validate the signature and prepare relevant error messages after the prefilters.