add memory filesystem for making tests faster, more hermetic, and more robust

Currently real files are used for file i/o during tests. An alternative I've used in the past is to have an in-memory representation of a file-system: https://github.com/apache/incubator-pagespeed-mod/blob/master/pagespeed/kernel/base/mem_file_system.h

This has these advantages:
* completely hermetic, with a clean slate between tests
* very fast (no I/O), speeding up the running of massive numbers of tests
* you can inject notifiers etc, so you can for example get a callback or condvar wakeup or similar when a file is written

Envoy already has a filesystem abstraction (yay!) and a way to squirrel that into code that needs to read/write files (Api::Api) just no in-memory impl for it.

I think this would be a good intern project. The example I gave above would not exactly meet Envoy's needs but it could be a reference point.