avoid intern calls on static strings
avoid intern calls on static strings

Summary:
Moves all `.intern()` calls in the flow generator on `&'static str` to constants.

I wish we had something that avoids the need for constants and could be statically compiled, but this should be a lot faster then a hash table lookup each time we need a `StringKey`.

Reviewed By: alunyov

Differential Revision: D21664520

fbshipit-source-id: dc9877ea7104202ca403323601dac5bdb7d6a5a3

wincent/relay,josephsavona/relay,facebook/relay,voideanvalue/relay,voideanvalue/relay,josephsavona/relay,atxwebs/relay,josephsavona/relay,wincent/relay,wincent/relay,yungsters/relay,wincent/relay,wincent/relay,voideanvalue/relay,atxwebs/relay,facebook/relay,facebook/relay,kassens/relay,xuorig/relay,kassens/relay,atxwebs/relay,atxwebs/relay,kassens/relay,kassens/relay,voideanvalue/relay,voideanvalue/relay,xuorig/relay,xuorig/relay,yungsters/relay,kassens/relay,xuorig/relay,kassens/relay,voideanvalue/relay,wincent/relay,facebook/relay,facebook/relay,yungsters/relay,xuorig/relay,facebook/relay,xuorig/relay