core: record individual messages with sizes to Census/tracing (#3461)

Two methods, outboundMessageSent() and inboundMessageRead() are added to StreamTracer in order to associate individual messages with sizes. Both types of sizes are optional, as allowed by Census tracing.

Both methods accept a sequence number as the type ID as required by Census. The original outboundMesage() and inboundMessage() are also replaced by overrides that take the sequence number, to better match the new methods. The deprecation of the old overrides are tracked by #3460