Add support for async getCacheConfig

[damassi]

This request came up while working through a caching exploration in artsy/reaction#3164. Basically, we would like to perform an intermediate request within getCacheConfig to determine if a particular route should be cached, and update cacheConfig appropriately. Right now, however, this function does not support promises.

Some breadcrumbs from found-relay:

• found-relay/src/Resolver.js

  Lines 34 to 38 in 824c5f6

  	const cacheConfigs = getRouteValues(
  	routeMatches,
  	route => route.getCacheConfig,
  	route => route.cacheConfig,
  	);

• found-relay/src/Resolver.js

  Line 146 in 824c5f6

  cacheConfig: cacheConfigs[i],