Refactors config to separate general config from flow config

[jlerche]

This PR gets the ball rolling for eventually implementing vhsd. When running vhs, it's essentially a single run, but with vhsd we can expect it to run indefinitely while running many flows. Starting up vhsd should not require any flow specific configuration, but will probably share a lot of config settings with a vhs run.

[ztreinhart]

Looks good to me. 👍

[andrewhare]

Nice work! A couple changes but looks good.

[andrewhare]

This looks really good, but I need one more change and it's my bad for not noticing this on the first round.

These config settings also need to be on FlowConfig:

vhs/session/config.go

Lines 7 to 11 in ef2581e

	Addr string
	CaptureResponse bool
	Middleware string
	TCPTimeout time.Duration
	HTTPTimeout time.Duration

vhs/session/config.go

Lines 15 to 18 in ef2581e

	GCSBucketName string
	GCSObjectName string
	BufferOutput bool

These config settings get used by the flow and can change from run to run.

[andrewhare]

Looks good 👍