Fixes https://emqx.atlassian.net/browse/EMQX-9635
During a sync call from process `A` to a buffer worker `B`, its call
to the underlying resource `C` can be very slow. In those cases, `A`
will receive a timeout response and expect no more messages from `B`
nor `C`. However, prior to this fix, if `B` is stuck in a long sync
call to `C` and then gets its response after `A` timed out, `B` would
still send the late response to `A`, polluting its mailbox.
Thales Macedo Garitezi