According to docs, `Server` uses provided error channel only to write
encountered error to it. In this case, there is no need to accept rw
channel to create `Server` instance. Strengthening the type to
write-only will allow the caller to ensure control of reading errors
from the provided channel.
The change is backward compatible since any `chan` is `chan<-`.
Signed-off-by: Leonard Lyubich <ctulhurider@gmail.com>
d09158161e