[commands] Copy HelpCommand instances to prevent race conditions.
[commands] Copy HelpCommand instances to prevent race conditions.

Fixes #2123

Slight breaking change if someone had an expectation that no copies
were made behind the scene (which is sensible), however writing code
that relies on this expectation is probably buggy anyway.

imayhaveborkedit/discord.py,rapptz/discord.py,Harmon758/discord.py,khazhyk/discord.py,Rapptz/discord.py,Harmon758/discord.py