Fix channelMap not being filled for bot users/>100 guilds
Fix channelMap not being filled for bot users/>100 guilds

Previously we only filled the channelMap inside of READY. Because of
lazy guild loading, this was not valid for the majority of users (e.g.
bot users and users with more than 100 guilds). We now properly handle
both cases, by filling in the channelMap on GUILD_CREATE.

This commit also refactors the way we do locking inside of GUILD_CREATE
and CHANNEL_CREATE to be a bit cleaner.

bwmarrin/discordgo,iopred/discordgo,robbix1206/discordgo,VagantemNumen/discordgo,jonas747/discordgo