Changes to caching process

[dhruv282]

• Histogram page fetches from the archive all the time to have latest mementos.

• Front-end requests an expected number of mementos which is compared to the number of cached.

• If the expected number matches with the cached number, carry on with the async series in processWithFileContents().

• If not, call updateCache(). This sequence of this function is as follows:

  • Fetch timemap from archive
  • Filter out cached mementos
  • Calculate a simhash on the non-cached mementos
  • Merge the non-cached with cached
  • Save this timemap to cache
  • Call appropriate functions based on the requested role