Provide a way so that DCP can be started from "now" without needing to query the server.

A discussion with the connectors revealed that they need to start DCP from "now". To do this they are dependent upon using "GetAllVbSeqnos" to retrieve sequence numbers to use as the start-seqno in stream-requests.

DCP stream-request already provides a mechanism that allows for a DCP stream to end "now" using flag 0x4

0x04 (Latest) - Specifies that the server should stream all mutations up to the current sequence number for that VBucket. The server will overwrite the value of the end sequence number field with the value of the latest sequence number.

Having an equivalent, but for "start=now" would be a useful addition and remove the need for connectors to use GetAllVBSeqnos just to start a stream.