Use memoizedPagechecker (#13191)
Use memoizedPagechecker (#13191)

It seems like this was supposed to use the memoized version of `pageChecker`. I also updated the function to memoize promises instead of promise results. Just in case this function ever gets called concurrently.
zeit/next.js,azukaru/next.js,JeromeFitz/next.js,zeit/next.js,azukaru/next.js,zeit/next.js,JeromeFitz/next.js,flybayer/next.js,flybayer/next.js,JeromeFitz/next.js,JeromeFitz/next.js,azukaru/next.js,flybayer/next.js,azukaru/next.js,flybayer/next.js