server tunables should be configurable

[davepacheco]

Right now, Dropshot has three hardcoded "tunables":

dropshot/dropshot/src/server.rs

Lines 134 to 139 in 96faa6f

	config: ServerConfig {
	/* We start aggressively to ensure test coverage. */
	request_body_max_bytes: 1024,
	page_max_nitems: NonZeroUsize::new(10000).unwrap(),
	page_default_nitems: NonZeroUsize::new(100).unwrap(),
	},

These should come from the ConfigDropshot struct, probably with default values.

[davepacheco]

Fixed by #64.