Various code cleanups

[Keitio]

• Added context.Context to most top-level functions
• Renamed errors to follow standard Go pattern
• Refactored interface{} to any
• Renamed method receivers to more meaningful names (self or this are bad practice in Go)
• Cleaned some select and switch uses
• Some small performance improvements (less channels created)
• Other various cleanups If this is too much change for one PR, I can split it to smaller ones