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Implement `ping-pong` for WebSocket server (#782)

* ws-server: Implement `ping-ping`
Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* ws-server: Set builder's ping_interval
Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* ws-server: Handle just `ping` frames
Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* ws-server: Simplify `select`
Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* Use `futures_util::select` instead of `select!` macro
Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* ws-server: Avoid pinning the delay
Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* ws-server: Log when a `Pong` frame is received
Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

* ws-server: Use tokio for submitting pings
Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>