iproto: send IPROTO_WATCH sync number in IPROTO_EVENT packet

We don't use this functionality in net.box (sync number is always 0 for
all watch/event packets), but other clients may actually use it.

Closes #8393

@TarantoolBot document
Title: Document that `IPROTO_EVENT` has sync number

Initially the sync number sent by a client in an `IPROTO_WATCH` request
was ignored and `IPROTO_EVENT` packet didn't have a sync number. There
were complaints about it from users so we consider this to be a bug.
Now the server sends the same sync number in an `IPROTO_EVENT` packet
as the one sent by the client in the last corresponding `IPROTO_WATCH`
request.