Do not call session onEvent as goroutine
Do not call session onEvent as goroutine

This is a stability improvement but may have a slight performance
impact. This change will be reviewed again later.  Doing this solves a
data race issue with the Sequence number that must be tracked for
gateway resume and heartbeats.  Event specific handlers are now called
as a goroutine though.

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