This CL implements two changes that must be logically connected.

The first is to provide a mechanism for disabling stream replay.
Previously, our documentation stated that if
StreamableServerTransport.EventStore was nil, a default in-memory event
store would be used. In that case, there would be no way to completely
disable stream replay, and as described in modelcontextprotocol#576, there are many cases
where replay is undesirable.

But of course changing the behavior of the transport zero value
implicitly affects its default behavior, and so this CL also implements
the proposal modelcontextprotocol#580: stream replay is disabled by default.

Implementing this change required a significant refactoring, as
previously we were relying on the event store for serialized message
delivery from the JSON-RPC layer to the MCP layer: the connection would
write to the event store, and independently the stream (be it an
incoming POST or replay GET) would iterate and serve messages in the
stream.

In order to achieve the goals of this CL, it was necessary to decouple
storage from delivery. The 'stream' abstraction now tracks a delivery
callback that writes straight to the HTTP response. It would have been
convenient to store the ongoing http.ResponseWriter directly in the
stream (this is how typescript does it), but due to the design of our
EventStore API, only the HTTP handler knows the next event index, so a
'deliver' abstraction was an unfortunate requirement (suggestions for
how to further simplify this are welcome).

More simplification is possible: in particular, as a result of this
refactoring it should be entirely possible to clean up streams once
we've received all responses. Any replay would only need access to the
EventStore, if at all. This is left to a follow-up CL, to limit this
already significant change.

Furthermore, a nice consequence of this refactoring is that, when not
using event storage, servers can get synchronous feedback that message
delivery failed, which should avoid unnecessary work. We can
additionally cancel ongoing requests on early client termination, but
that is also left to a follow-up CL.

Throughout, the terminology 'standalone SSE stream' replaced 'hanging
GET' when referring to the non-replay GET request issued by the client.
This is consistent with other SDKs.

Fixes modelcontextprotocol#576
Updates modelcontextprotocol#580