pkg/stream: Use pointer receiver for Basic struct
pkg/stream: Use pointer receiver for Basic struct

This avoids copying the struct contents for every method call, which
can cause data races.

Closes #3157

Signed-off-by: Jonathan Rudenberg <3692bfa45759a67d83aedf0045f6cb635a966abf@titanous.com>

philiplb/flynn,flynn/flynn,lmars/flynn,philiplb/flynn,philiplb/flynn,philiplb/flynn,flynn/flynn,philiplb/flynn,lmars/flynn,flynn/flynn,lmars/flynn,flynn/flynn,flynn/flynn,lmars/flynn,lmars/flynn