[Issue 5324][go client] Use Buffered channels to avoid blocking on callback

[hrsakai]

Motivation

• In order to avoid blocking a thread on callback by using buffered channels.
• the root cause of [go client] Producer::Close() may get stuck #5324 seems to be in cpp client library(Callbacks must be executed on another thread), but here I modified go client library.
• In another PR, we will consider fixing the cpp client library.

Modifications

• Use buffered channels in methods that call async methods.

[hrsakai]

rerun integration tests