Tell a WebSocket read timeout apart from a closed connection

read() collapsed every failure into Fail and marked the connection closed with
it, so a read timeout could not be used to get control back and send on the
same connection -- it killed the connection instead. The information was
already there and thrown away: SocketStream::read records Error::Timeout, and
read_websocket_frame flattened it into a bool.

ReadResult gains Timeout, reported only when the timeout elapsed on a frame
boundary with nothing consumed, which is the only case where the stream can be
read again. Every multi-byte field now loops until it has its bytes, which also
fixes a frame header straddling the read buffer's boundary failing the frame:
Stream::read is allowed to return less than asked for, and only the payload
was reading in a loop.

ws::WebSocket::set_read_timeout() lets a server handler bound its own reads,
and WebSocketClient::set_read_timeout() now reaches an already-open connection
instead of only seeding the next connect().

The read timeout macro splits in two. A client waits forever by default -- a
read timeout is the caller's tool for taking back control, not a liveness
check, which is ping/pong's job -- while a server keeps the 300s that reclaims
a worker from a peer gone quiet. Defining the old name still sets both.

Also record a reason on the two WebSocketSSLStream::read failure paths that
returned -1 without one, so get_error() cannot report a previous call's
timeout, and make SocketStream's read timeout atomic now that it can be
changed while a read is in flight.

docs-src/pages/en/cookbook/w01-websocket-echo.md

@@ -36,6 +36,7 @@ The `read()` return value is a `ReadResult` enum:
 - `ReadResult::Text`: received a text message
 - `ReadResult::Binary`: received a binary message
 - `ReadResult::Fail`: error, or connection closed
+- `ReadResult::Timeout`: the read timeout elapsed with nothing received; the connection is still open. Only appears once a read timeout is set — see [W06. Set Timeouts](../w06-websocket-timeouts)
 
 ## Client: talk to the echo server