Add commandline flag to start with syncer turned off

[lrettig]

This allows an API usage pattern—used to extract all mesh/state data from a full node—where a client starts a node with one or more GRPC services enabled, opens streams to listen to the API, then manually enables syncing via NodeService.SyncStart. By starting the node with sync turned off, then manually enabling it only after subscribing to the relevant streams, the client can be sure they don't miss any data as the node syncs/streams data.